Explain Conservation of Mass - Chemistry

1. Iron reacts with oxygen in air to form rust (iron(III) oxide): $4\text{Fe}(s) + 3\text{O}_2(g) \rightarrow 2\text{Fe}_2\text{O}_3(s)$ A piece of steel wool has a mass of 20 g. After being left in moist air for several days, the rusted wool has a mass of 27 g. Which statement best explains the mass increase?
   Answer: Oxygen from the air combined with the iron; if you included the oxygen taken from the air, total mass would be conserved. This question tests your understanding of the law of conservation of mass—the principle that mass is neither created nor destroyed in chemical reactions, only rearranged as atoms reorganize into different substances. The law of conservation of mass states that the total mass of all reactants must equal the total mass of all products because atoms are not created or destroyed in chemical reactions, merely rearranged: if you start with 50 atoms of various types (in molecules as reactants), you end with those same 50 atoms (now in molecules as products), and since mass comes from atoms, the total mass stays constant. In this rusting reaction, the steel wool increased from 20 g to 27 g (gaining 7 g) because oxygen gas from the air combined with the iron atoms to form iron oxide (rust)—those 7 g came from oxygen atoms that were originally in the air and are now chemically bonded to the iron. Choice B correctly applies conservation of mass by recognizing that in an open system, substances can react with gases from the air, and the mass increase comes from incorporating oxygen atoms that have mass. Choice A incorrectly claims new matter was created (violating conservation), Choice C wrongly links color change to mass gain, and Choice D incorrectly connects volume to mass changes. Using conservation of mass in open systems: When mass appears to increase, look for gas absorption—here, oxygen from air combining with iron explains the 7 g gain. The total mass is conserved: 20 g iron + 7 g oxygen (from air) = 27 g iron oxide, accounting for all atoms involved.
2. A student reacts 15 g of metal M with 10 g of chlorine gas in a sealed container to form metal chloride: $\mathrm{M(s) + Cl_2(g) \rightarrow MCl_2(s)}$. After the reaction, all the chlorine has reacted and no gas remains. What should be the mass of the solid product $\mathrm{MCl_2}$?
   Answer: 25 g, because the masses of reactants add up and matter is conserved in a closed system. This question tests your understanding of the law of conservation of mass—the principle that mass is neither created nor destroyed in chemical reactions, only rearranged as atoms reorganize into different substances. The law of conservation of mass states that the total mass of all reactants must equal the total mass of all products because atoms are not created or destroyed in chemical reactions, merely rearranged: if you start with 50 atoms of various types (in molecules as reactants), you end with those same 50 atoms (now in molecules as products), and since mass comes from atoms, the total mass stays constant. This is why balanced equations work—they ensure atom counts match on both sides, which guarantees mass conservation. However, in OPEN systems where gases can escape or be absorbed from the air, the MEASURED mass may appear to change even though total mass is actually conserved—you just have to account for gases that left or entered the system! In this sealed container (closed system), 15 g M + 10 g Cl2 = 25 g total reactants, and since all react to form solid MCl2 with no gas left, the product mass should be 25 g. Choice C correctly applies conservation of mass by recognizing that in a closed system, the product mass equals the sum of reactant masses at 25 g. For example, choice A fails because it suggests the gas disappears without mass, but gases have mass, and here it's incorporated into the solid, conserving total mass. Using conservation of mass: (1) In CLOSED systems (sealed container, nothing escapes): total mass before = total mass after, always! Add all reactant masses, add all product masses, they should match exactly. If they don't in data, measurement error occurred. (2) In OPEN systems (reaction in open air, unsealed): APPARENT mass may change because gases escape or enter. Mass INCREASE: gas from air absorbed (oxygen combining with substance during burning, rusting). Mass DECREASE: gas released to air (CO2, H2O vapor from combustion escaping). True total mass (including gases) still conserved, but you need to account for the gas! (3) To verify conservation: list ALL substances including gases. Example: burning 10g wood in open air leaves 1g ash—where did 9g go? Answer: 9g became CO2 and H2O vapor (gases escaped). Total: 10g wood + oxygen from air → 1g ash + 9g gases. Mass conserved when all counted! The "why mass appears to change" explanation: (1) Identify if system is open or closed. (2) If closed and mass changes in data, error occurred (conservation violated only by measurement mistakes). (3) If open and mass increases, look for gas absorption (combining with oxygen from air is most common). (4) If open and mass decreases, look for gas release (CO2, H2O vapor, or other gases escaping). (5) Explain: "Mass appears to decrease but is actually conserved because [specific gas] escaped; if measured in closed system, that gas would be captured and total mass would be constant." This accounting explains apparent violations while affirming conservation!
3. A student burns a 25 g piece of charcoal (carbon) in an open grill. Afterward, only 2 g of ash remains in the grill. Which is the best explanation for the large apparent mass decrease?
   Answer: Most of the products were gases (like $\text{CO}_2$) that escaped into the air, so they were not included in the measured mass. This question tests your understanding of the law of conservation of mass—the principle that mass is neither created nor destroyed in chemical reactions, only rearranged as atoms reorganize into different substances. The law of conservation of mass states that the total mass of all reactants must equal the total mass of all products because atoms are not created or destroyed in chemical reactions, merely rearranged: if you start with 50 atoms of various types (in molecules as reactants), you end with those same 50 atoms (now in molecules as products), and since mass comes from atoms, the total mass stays constant. This is why balanced equations work—they ensure atom counts match on both sides, which guarantees mass conservation. However, in OPEN systems where gases can escape or be absorbed from the air, the MEASURED mass may appear to change even though total mass is actually conserved—you just have to account for gases that left or entered the system! In this open grill, the 25 g charcoal reacts with oxygen from air to form mostly CO2 gas (escaping) and 2 g ash, so the 23 g difference is in the escaped gases, but total mass including them and the oxygen input conserves the overall mass. Choice B correctly applies conservation of mass by recognizing that in open systems, apparent decreases occur when gaseous products like CO2 escape and are not measured. Choice A fails by claiming atoms are destroyed, which violates the law since carbon atoms rearrange into CO2; choices C and D incorrectly attribute the change to heat or volume, but heat lacks mass, and volume doesn't affect mass conservation. Using conservation of mass: (1) In CLOSED systems (sealed container, nothing escapes): total mass before = total mass after, always! Add all reactant masses, add all product masses, they should match exactly. If they don't in data, measurement error occurred. (2) In OPEN systems (reaction in open air, unsealed): APPARENT mass may change because gases escape or enter. Mass INCREASE: gas from air absorbed (oxygen combining with substance during burning, rusting). Mass DECREASE: gas released to air (CO2, H2O vapor from combustion escaping). True total mass (including gases) still conserved, but you need to account for the gas! (3) To verify conservation: list ALL substances including gases. Example: burning 10g wood in open air leaves 1g ash—where did 9g go? Answer: 9g became CO2 and H2O vapor (gases escaped). Total: 10g wood + oxygen from air → 1g ash + 9g gases. Mass conserved when all counted! The "why mass appears to change" explanation: (1) Identify if system is open or closed. (2) If closed and mass changes in data, error occurred (conservation violated only by measurement mistakes). (3) If open and mass increases, look for gas absorption (combining with oxygen from air is most common). (4) If open and mass decreases, look for gas release (CO2, H2O vapor, or other gases escaping). (5) Explain: "Mass appears to decrease but is actually conserved because [specific gas] escaped; if measured in closed system, that gas would be captured and total mass would be constant." This accounting explains apparent violations while affirming conservation!
4. In a sealed plastic bag, a student combines 12 g of solution A and 8 g of solution B. A solid precipitate forms (A(aq) + B(aq) → AB(s)). The mass of the sealed bag and its contents is measured: - Before mixing: 120 g - After reaction: 120 g Which conclusion best matches these data?
   Answer: The mass stayed the same because atoms were rearranged but not created or destroyed in the sealed system. This question tests your understanding of the law of conservation of mass—the principle that mass is neither created nor destroyed in chemical reactions, only rearranged as atoms reorganize into different substances. The law of conservation of mass states that the total mass of all reactants must equal the total mass of all products because atoms are not created or destroyed in chemical reactions, merely rearranged: if you start with 50 atoms of various types (in molecules as reactants), you end with those same 50 atoms (now in molecules as products), and since mass comes from atoms, the total mass stays constant. This is why balanced equations work—they ensure atom counts match on both sides, which guarantees mass conservation. However, in OPEN systems where gases can escape or be absorbed from the air, the MEASURED mass may appear to change even though total mass is actually conserved—you just have to account for gases that left or entered the system! Here, the sealed bag ensures a closed system where nothing can escape or enter, so the total mass remains 120 g before and after the reaction, as the 12 g of A and 8 g of B (total 20 g contents plus 100 g bag) rearrange into the precipitate without any loss or gain. Choice A correctly applies conservation of mass by recognizing that in a sealed system, mass stays the same due to atomic rearrangement without creation or destruction. Choice C fails because it claims no reaction occurred, but the formation of a solid precipitate indicates a chemical change did happen; conservation holds regardless as long as the system is closed. Using conservation of mass: (1) In CLOSED systems (sealed container, nothing escapes): total mass before = total mass after, always! Add all reactant masses, add all product masses, they should match exactly. If they don't in data, measurement error occurred. (2) In OPEN systems (reaction in open air, unsealed): APPARENT mass may change because gases escape or enter. Mass INCREASE: gas from air absorbed (oxygen combining with substance during burning, rusting). Mass DECREASE: gas released to air (CO2, H2O vapor from combustion escaping). True total mass (including gases) still conserved, but you need to account for the gas! (3) To verify conservation: list ALL substances including gases. Example: burning 10g wood in open air leaves 1g ash—where did 9g go? Answer: 9g became CO2 and H2O vapor (gases escaped). Total: 10g wood + oxygen from air → 1g ash + 9g gases. Mass conserved when all counted! The "why mass appears to change" explanation: (1) Identify if system is open or closed. (2) If closed and mass changes in data, error occurred (conservation violated only by measurement mistakes). (3) If open and mass increases, look for gas absorption (combining with oxygen from air is most common). (4) If open and mass decreases, look for gas release (CO2, H2O vapor, or other gases escaping). (5) Explain: "Mass appears to decrease but is actually conserved because [specific gas] escaped; if measured in closed system, that gas would be captured and total mass would be constant." This accounting explains apparent violations while affirming conservation! You're doing great—remember, sealed systems make conservation easy to observe!
5. Hydrogen and oxygen react in a sealed container: $\mathrm{2H_2(g) + O_2(g) \rightarrow 2H_2O(l)}$. A student starts with 8 g of hydrogen gas and 64 g of oxygen gas in the sealed container. After the reaction goes to completion, what should be the mass of liquid water formed (ignoring the mass of the container)?
   Answer: 72 g, because total mass of reactants equals total mass of products in a closed system. This question tests your understanding of the law of conservation of mass—the principle that mass is neither created nor destroyed in chemical reactions, only rearranged as atoms reorganize into different substances. The law of conservation of mass states that the total mass of all reactants must equal the total mass of all products because atoms are not created or destroyed in chemical reactions, merely rearranged: if you start with 50 atoms of various types (in molecules as reactants), you end with those same 50 atoms (now in molecules as products), and since mass comes from atoms, the total mass stays constant. This is why balanced equations work—they ensure atom counts match on both sides, which guarantees mass conservation. However, in OPEN systems where gases can escape or be absorbed from the air, the MEASURED mass may appear to change even though total mass is actually conserved—you just have to account for gases that left or entered the system! In this sealed container (closed system), 8 g H2 + 64 g O2 = 72 g total reactants, and the reaction produces 72 g H2O liquid, with no leftover gases since it's stoichiometric. Choice B correctly applies conservation of mass by recognizing that in a closed system, the mass of products equals the mass of reactants at 72 g. For example, choice A fails because it incorrectly assumes gases weigh less than liquids, but mass is independent of state and is conserved regardless. Using conservation of mass: (1) In CLOSED systems (sealed container, nothing escapes): total mass before = total mass after, always! Add all reactant masses, add all product masses, they should match exactly. If they don't in data, measurement error occurred. (2) In OPEN systems (reaction in open air, unsealed): APPARENT mass may change because gases escape or enter. Mass INCREASE: gas from air absorbed (oxygen combining with substance during burning, rusting). Mass DECREASE: gas released to air (CO2, H2O vapor from combustion escaping). True total mass (including gases) still conserved, but you need to account for the gas! (3) To verify conservation: list ALL substances including gases. Example: burning 10g wood in open air leaves 1g ash—where did 9g go? Answer: 9g became CO2 and H2O vapor (gases escaped). Total: 10g wood + oxygen from air → 1g ash + 9g gases. Mass conserved when all counted! The "why mass appears to change" explanation: (1) Identify if system is open or closed. (2) If closed and mass changes in data, error occurred (conservation violated only by measurement mistakes). (3) If open and mass increases, look for gas absorption (combining with oxygen from air is most common). (4) If open and mass decreases, look for gas release (CO2, H2O vapor, or other gases escaping). (5) Explain: "Mass appears to decrease but is actually conserved because [specific gas] escaped; if measured in closed system, that gas would be captured and total mass would be constant." This accounting explains apparent violations while affirming conservation!
6. Hydrogen reacts with oxygen in a sealed container: $2\text{H}_2(g) + \text{O}_2(g) \rightarrow 2\text{H}_2\text{O}(g)$ A student starts with 6 g of $\text{H}_2$ and 48 g of $\text{O}_2$ in the sealed container. After the reaction is complete, what should be the total mass of all substances in the container?
   Answer: 54 g This question tests your understanding of the law of conservation of mass—the principle that mass is neither created nor destroyed in chemical reactions, only rearranged as atoms reorganize into different substances. The law of conservation of mass states that the total mass of all reactants must equal the total mass of all products because atoms are not created or destroyed in chemical reactions, merely rearranged: if you start with 50 atoms of various types (in molecules as reactants), you end with those same 50 atoms (now in molecules as products), and since mass comes from atoms, the total mass stays constant. In this sealed container, we start with 6 g H₂ + 48 g O₂ = 54 g total mass of reactants, and since the container is sealed (closed system), no matter can enter or leave, so the total mass after reaction must still be 54 g. Choice C correctly applies conservation of mass: whether the substances exist as separate H₂ and O₂ or combined as H₂O, the same atoms are present with the same total mass of 54 g. The other choices violate conservation—42 g (A) and 48 g (B) suggest mass was lost somehow, while 6 g (D) absurdly suggests most mass vanished. Using conservation of mass: (1) In CLOSED systems (sealed container, nothing escapes): total mass before = total mass after, always! Add all reactant masses (6 + 48 = 54 g), and that equals the total mass after reaction. (2) It doesn't matter what products form or what state they're in—gas, liquid, or solid—the total mass remains constant because the same atoms are just rearranged.
7. A student heats 25 g of limestone (calcium carbonate) in an open container. Reaction: $\mathrm{CaCO_3(s) \rightarrow CaO(s) + CO_2(g)}$. After heating, 14 g of solid calcium oxide remains in the container. Which statement best accounts for the missing mass?
   Answer: 11 g of $\mathrm{CO_2}$ gas left the open container, so the measured mass decreased. This question tests your understanding of the law of conservation of mass—the principle that mass is neither created nor destroyed in chemical reactions, only rearranged as atoms reorganize into different substances. The law of conservation of mass states that the total mass of all reactants must equal the total mass of all products because atoms are not created or destroyed in chemical reactions, merely rearranged: if you start with 50 atoms of various types (in molecules as reactants), you end with those same 50 atoms (now in molecules as products), and since mass comes from atoms, the total mass stays constant. This is why balanced equations work—they ensure atom counts match on both sides, which guarantees mass conservation. However, in OPEN systems where gases can escape or be absorbed from the air, the MEASURED mass may appear to change even though total mass is actually conserved—you just have to account for gases that left or entered the system! In this open container, heating 25 g $\mathrm{CaCO_3}$ produces 14 g $\mathrm{CaO}$ and 11 g $\mathrm{CO_2}$ gas, which escapes, so the measured mass decreases, but total mass including the escaped $\mathrm{CO_2}$ would be 25 g. Choice A correctly applies conservation of mass by recognizing that in an open system, 11 g of $\mathrm{CO_2}$ escaped, accounting for the missing mass. For example, choice B fails because it claims mass was destroyed, but conservation states mass is never destroyed, only rearranged, and the gas has mass that left the system. Using conservation of mass: (1) In CLOSED systems (sealed container, nothing escapes): total mass before = total mass after, always! Add all reactant masses, add all product masses, they should match exactly. If they don't in data, measurement error occurred. (2) In OPEN systems (reaction in open air, unsealed): APPARENT mass may change because gases escape or enter. Mass INCREASE: gas from air absorbed (oxygen combining with substance during burning, rusting). Mass DECREASE: gas released to air ($\mathrm{CO_2}$, $\mathrm{H_2O}$ vapor from combustion escaping). True total mass (including gases) still conserved, but you need to account for the gas! (3) To verify conservation: list ALL substances including gases. Example: burning 10g wood in open air leaves 1g ash—where did 9g go? Answer: 9g became $\mathrm{CO_2}$ and $\mathrm{H_2O}$ vapor (gases escaped). Total: 10g wood + oxygen from air → 1g ash + 9g gases. Mass conserved when all counted! The "why mass appears to change" explanation: (1) Identify if system is open or closed. (2) If closed and mass changes in data, error occurred (conservation violated only by measurement mistakes). (3) If open and mass increases, look for gas absorption (combining with oxygen from air is most common). (4) If open and mass decreases, look for gas release ($\mathrm{CO_2}$, $\mathrm{H_2O}$ vapor, or other gases escaping). (5) Explain: "Mass appears to decrease but is actually conserved because [specific gas] escaped; if measured in closed system, that gas would be captured and total mass would be constant." This accounting explains apparent violations while affirming conservation!
8. In a sealed container, 10 g of carbon reacts completely with 30 g of oxygen to form carbon dioxide: $\text{C}(s) + \text{O}_2(g) \rightarrow \text{CO}_2(g)$ After the reaction, what should be the total mass of gases and solids in the container?
   Answer: 40 g This question tests your understanding of the law of conservation of mass—the principle that mass is neither created nor destroyed in chemical reactions, only rearranged as atoms reorganize into different substances. The law of conservation of mass states that the total mass of all reactants must equal the total mass of all products because atoms are not created or destroyed in chemical reactions, merely rearranged: if you start with 50 atoms of various types (in molecules as reactants), you end with those same 50 atoms (now in molecules as products), and since mass comes from atoms, the total mass stays constant. In this sealed container reaction, we start with 10 g carbon + 30 g oxygen = 40 g total mass of reactants, and since the container is sealed (closed system), all atoms remain inside as CO₂ gas, giving a total mass of 40 g after reaction. Choice D correctly applies conservation of mass: whether the carbon and oxygen exist separately or combined as CO₂, the same atoms are present with the same total mass of 40 g in the sealed container. The other choices (20 g, 30 g, 10 g) would require some mass to vanish, violating conservation. Using conservation of mass: (1) In CLOSED systems (sealed container, nothing escapes): total mass before = total mass after, always! (2) State changes don't matter—whether reactants are solid carbon and gaseous oxygen, or the product is gaseous CO₂, the total mass remains 10 + 30 = 40 g because the same atoms are present, just rearranged into new molecules.
9. A student performs the same gas-producing reaction two ways: Reaction: X(aq) + Y(aq) → Z(aq) + CO$_2$(g) - Trial 1 (open cup): total mass before = 150 g; total mass after = 146 g - Trial 2 (sealed flask): total mass before = 150 g; total mass after = 150 g Which statement best explains the different results?
   Answer: In Trial 1, CO$_2$ escaped to the air; in Trial 2, CO$_2$ was trapped, so the measured mass stayed constant. This question tests your understanding of the law of conservation of mass—the principle that mass is neither created nor destroyed in chemical reactions, only rearranged as atoms reorganize into different substances. The law of conservation of mass states that the total mass of all reactants must equal the total mass of all products because atoms are not created or destroyed in chemical reactions, merely rearranged: if you start with 50 atoms of various types (in molecules as reactants), you end with those same 50 atoms (now in molecules as products), and since mass comes from atoms, the total mass stays constant. This is why balanced equations work—they ensure atom counts match on both sides, which guarantees mass conservation. However, in OPEN systems where gases can escape or be absorbed from the air, the MEASURED mass may appear to change even though total mass is actually conserved—you just have to account for gases that left or entered the system! In Trial 1 (open), CO2 gas escapes, causing a 4 g decrease from 150 g to 146 g, while in Trial 2 (sealed), the gas is trapped, keeping the mass at 150 g; total mass is conserved in both, but the open system shows apparent loss. Choice B correctly applies conservation of mass by recognizing the role of the open vs. closed system in whether gas escape affects measured mass. Choice A fails because it suggests atoms were destroyed in Trial 1, violating conservation—in reality, atoms are conserved, but the gas left the measured system in the open trial. Using conservation of mass: (1) In CLOSED systems (sealed container, nothing escapes): total mass before = total mass after, always! Add all reactant masses, add all product masses, they should match exactly. If they don't in data, measurement error occurred. (2) In OPEN systems (reaction in open air, unsealed): APPARENT mass may change because gases escape or enter. Mass INCREASE: gas from air absorbed (oxygen combining with substance during burning, rusting). Mass DECREASE: gas released to air (CO2, H2O vapor from combustion escaping). True total mass (including gases) still conserved, but you need to account for the gas! (3) To verify conservation: list ALL substances including gases. Example: burning 10g wood in open air leaves 1g ash—where did 9g go? Answer: 9g became CO2 and H2O vapor (gases escaped). Total: 10g wood + oxygen from air → 1g ash + 9g gases. Mass conserved when all counted! The "why mass appears to change" explanation: (1) Identify if system is open or closed. (2) If closed and mass changes in data, error occurred (conservation violated only by measurement mistakes). (3) If open and mass increases, look for gas absorption (combining with oxygen from air is most common). (4) If open and mass decreases, look for gas release (CO2, H2O vapor, or other gases escaping). (5) Explain: "Mass appears to decrease but is actually conserved because [specific gas] escaped; if measured in closed system, that gas would be captured and total mass would be constant." This accounting explains apparent violations while affirming conservation! Terrific comparison of trials—you've got this!
10. A reaction is carried out in a sealed flask: $\text{A}(aq) + \text{B}(aq) \rightarrow \text{C}(aq)$ Mass of A solution added: 18 g Mass of B solution added: 7 g After the reaction, the flask contains only solution C (no gas formed) and remains sealed. What should be the mass of solution C inside the flask (ignoring the flask's mass)?
    Answer: 25 g This question tests your understanding of the law of conservation of mass—the principle that mass is neither created nor destroyed in chemical reactions, only rearranged as atoms reorganize into different substances. The law of conservation of mass states that the total mass of all reactants must equal the total mass of all products because atoms are not created or destroyed in chemical reactions, merely rearranged: if you start with 50 atoms of various types (in molecules as reactants), you end with those same 50 atoms (now in molecules as products), and since mass comes from atoms, the total mass stays constant. In this sealed flask reaction, we have 18 g of solution A + 7 g of solution B = 25 g total mass of reactants, and since the flask is sealed (closed system) with no gas formation mentioned, all atoms remain in the flask as solution C, which must have mass = 25 g. Choice C correctly applies conservation of mass: in a closed system where all products remain inside, the total mass of products equals the total mass of reactants (18 + 7 = 25 g). The other choices (11 g, 18 g, 7 g) would require mass to disappear somehow, violating conservation. Using conservation of mass: (1) In CLOSED systems (sealed container, nothing escapes): total mass before = total mass after, always! (2) For solutions mixing and reacting: add all solution masses before reaction, and that sum equals the total mass after reaction, regardless of what new compounds form in solution.
11. A student mixes baking soda and vinegar in an open beaker. The reaction is: NaHCO$_3$(s) + CH$_3$COOH(aq) → CH$_3$COONa(aq) + CO$_2$(g) + H$_2$O(l). Masses measured: - Before: beaker + contents = 200 g - After bubbling stops: beaker + contents = 196 g Which statement best explains the 4 g decrease?
    Answer: CO$_2$(g) escaped from the open beaker, so the measured mass decreased even though total mass would be conserved in a closed system. This question tests your understanding of the law of conservation of mass—the principle that mass is neither created nor destroyed in chemical reactions, only rearranged as atoms reorganize into different substances. The law of conservation of mass states that the total mass of all reactants must equal the total mass of all products because atoms are not created or destroyed in chemical reactions, merely rearranged: if you start with 50 atoms of various types (in molecules as reactants), you end with those same 50 atoms (now in molecules as products), and since mass comes from atoms, the total mass stays constant. This is why balanced equations work—they ensure atom counts match on both sides, which guarantees mass conservation. However, in OPEN systems where gases can escape or be absorbed from the air, the MEASURED mass may appear to change even though total mass is actually conserved—you just have to account for gases that left or entered the system! In this experiment, the reaction produces CO2 gas in an open beaker, so the initial 200 g includes the reactants, but after the reaction, 4 g of CO2 escapes into the air, leading to a measured mass of 196 g; if the system were closed, the total mass would remain 200 g including the trapped gas. Choice C correctly applies conservation of mass by recognizing that total mass is conserved but the measured mass decreases due to the escaping CO2 gas in the open system. Choice A fails because it suggests mass was destroyed, which violates conservation—gas has mass, it's just not lighter in a way that destroys matter; instead, the gas simply left the beaker. Using conservation of mass: (1) In CLOSED systems (sealed container, nothing escapes): total mass before = total mass after, always! Add all reactant masses, add all product masses, they should match exactly. If they don't in data, measurement error occurred. (2) In OPEN systems (reaction in open air, unsealed): APPARENT mass may change because gases escape or enter. Mass INCREASE: gas from air absorbed (oxygen combining with substance during burning, rusting). Mass DECREASE: gas released to air (CO2, H2O vapor from combustion escaping). True total mass (including gases) still conserved, but you need to account for the gas! (3) To verify conservation: list ALL substances including gases. Example: burning 10g wood in open air leaves 1g ash—where did 9g go? Answer: 9g became CO2 and H2O vapor (gases escaped). Total: 10g wood + oxygen from air → 1g ash + 9g gases. Mass conserved when all counted! The "why mass appears to change" explanation: (1) Identify if system is open or closed. (2) If closed and mass changes in data, error occurred (conservation violated only by measurement mistakes). (3) If open and mass increases, look for gas absorption (combining with oxygen from air is most common). (4) If open and mass decreases, look for gas release (CO2, H2O vapor, or other gases escaping). (5) Explain: "Mass appears to decrease but is actually conserved because [specific gas] escaped; if measured in closed system, that gas would be captured and total mass would be constant." This accounting explains apparent violations while affirming conservation! Keep practicing these open vs. closed system distinctions, and you'll master conservation of mass in no time!
12. Nitrogen and hydrogen form ammonia in a sealed container: $\text{N}_2(g) + 3\text{H}_2(g) \rightarrow 2\text{NH}_3(g)$. If the container holds 18 g of $\text{N}_2$ and 6 g of $\text{H}_2$ before the reaction, what should be the total mass of gases in the container after the reaction (assuming the container stays sealed and the reaction goes to completion)?
    Answer: 24 g, because total mass is conserved in a closed system. This question tests your understanding of the law of conservation of mass—the principle that mass is neither created nor destroyed in chemical reactions, only rearranged as atoms reorganize into different substances. The law of conservation of mass states that the total mass of all reactants must equal the total mass of all products because atoms are not created or destroyed in chemical reactions, merely rearranged: if you start with 50 atoms of various types (in molecules as reactants), you end with those same 50 atoms (now in molecules as products), and since mass comes from atoms, the total mass stays constant. This is why balanced equations work—they ensure atom counts match on both sides, which guarantees mass conservation. However, in OPEN systems where gases can escape or be absorbed from the air, the MEASURED mass may appear to change even though total mass is actually conserved—you just have to account for gases that left or entered the system! In this sealed container, the initial gases total 18 g N2 + 6 g H2 = 24 g, and after forming NH3, the mass remains 24 g because it's closed, with all atoms rearranged into the product gas. Choice B correctly applies conservation of mass by recognizing that in closed systems, total mass is conserved regardless of the reaction's completion or the gases involved. Choice A fails by linking mass to molecule count, but mass depends on atoms, not molecules; choice D incorrectly adds mass from energy, yet energy doesn't contribute to mass in chemical reactions, and choice C picks an arbitrary value without conservation reasoning. Using conservation of mass: (1) In CLOSED systems (sealed container, nothing escapes): total mass before = total mass after, always! Add all reactant masses, add all product masses, they should match exactly. If they don't in data, measurement error occurred. (2) In OPEN systems (reaction in open air, unsealed): APPARENT mass may change because gases escape or enter. Mass INCREASE: gas from air absorbed (oxygen combining with substance during burning, rusting). Mass DECREASE: gas released to air (CO2, H2O vapor from combustion escaping). True total mass (including gases) still conserved, but you need to account for the gas! (3) To verify conservation: list ALL substances including gases. Example: burning 10g wood in open air leaves 1g ash—where did 9g go? Answer: 9g became CO2 and H2O vapor (gases escaped). Total: 10g wood + oxygen from air → 1g ash + 9g gases. Mass conserved when all counted! The "why mass appears to change" explanation: (1) Identify if system is open or closed. (2) If closed and mass changes in data, error occurred (conservation violated only by measurement mistakes). (3) If open and mass increases, look for gas absorption (combining with oxygen from air is most common). (4) If open and mass decreases, look for gas release (CO2, H2O vapor, or other gases escaping). (5) Explain: "Mass appears to decrease but is actually conserved because [specific gas] escaped; if measured in closed system, that gas would be captured and total mass would be constant." This accounting explains apparent violations while affirming conservation!
13. In a sealed flask, hydrogen reacts with oxygen: $2\text{H}_2(g) + \text{O}_2(g) \rightarrow 2\text{H}_2\text{O}(g)$. The total mass of the sealed flask and gases is 200 g before the reaction and 200 g after the reaction. What is the best conclusion?
    Answer: Mass is conserved in a closed system because atoms are rearranged but not created or destroyed. This question tests your understanding of the law of conservation of mass—the principle that mass is neither created nor destroyed in chemical reactions, only rearranged as atoms reorganize into different substances. The law of conservation of mass states that the total mass of all reactants must equal the total mass of all products because atoms are not created or destroyed in chemical reactions, merely rearranged: if you start with 50 atoms of various types (in molecules as reactants), you end with those same 50 atoms (now in molecules as products), and since mass comes from atoms, the total mass stays constant. This is why balanced equations work—they ensure atom counts match on both sides, which guarantees mass conservation. However, in OPEN systems where gases can escape or be absorbed from the air, the MEASURED mass may appear to change even though total mass is actually conserved—you just have to account for gases that left or entered the system! In this sealed flask, the mass remains 200 g before and after because it's a closed system where all reactants (H2 and O2 gases) are converted to H2O gas, with no material escaping, so the atoms are simply rearranged into the product while total mass stays constant. Choice A correctly applies conservation of mass by recognizing that in closed systems, mass is conserved through atom rearrangement without creation or destruction. Choice B fails by assuming no gas production explains the constant mass, but the reaction does produce gas (H2O vapor), yet mass stays the same because the system is sealed; choices C and D incorrectly tie mass to molecule count or energy conversion, but mass conservation depends on atom count, not molecules or energy-mass equivalence in typical reactions. Using conservation of mass: (1) In CLOSED systems (sealed container, nothing escapes): total mass before = total mass after, always! Add all reactant masses, add all product masses, they should match exactly. If they don't in data, measurement error occurred. (2) In OPEN systems (reaction in open air, unsealed): APPARENT mass may change because gases escape or enter. Mass INCREASE: gas from air absorbed (oxygen combining with substance during burning, rusting). Mass DECREASE: gas released to air (CO2, H2O vapor from combustion escaping). True total mass (including gases) still conserved, but you need to account for the gas! (3) To verify conservation: list ALL substances including gases. Example: burning 10g wood in open air leaves 1g ash—where did 9g go? Answer: 9g became CO2 and H2O vapor (gases escaped). Total: 10g wood + oxygen from air → 1g ash + 9g gases. Mass conserved when all counted! The "why mass appears to change" explanation: (1) Identify if system is open or closed. (2) If closed and mass changes in data, error occurred (conservation violated only by measurement mistakes). (3) If open and mass increases, look for gas absorption (combining with oxygen from air is most common). (4) If open and mass decreases, look for gas release (CO2, H2O vapor, or other gases escaping). (5) Explain: "Mass appears to decrease but is actually conserved because [specific gas] escaped; if measured in closed system, that gas would be captured and total mass would be constant." This accounting explains apparent violations while affirming conservation!
14. A student claims: "Because the number of molecules changes during a reaction, the total mass must change too." Consider the reaction in a sealed container: 2H$_2$(g) + O$_2$(g) → 2H$_2$O(l). The container's total mass is 80 g before and 80 g after. Which choice best addresses the student's claim using conservation ideas?
    Answer: Mass is conserved because the same numbers of H and O atoms are present before and after; they are just rearranged into different molecules. This question tests your understanding of the law of conservation of mass—the principle that mass is neither created nor destroyed in chemical reactions, only rearranged as atoms reorganize into different substances. The law of conservation of mass states that the total mass of all reactants must equal the total mass of all products because atoms are not created or destroyed in chemical reactions, merely rearranged: if you start with 50 atoms of various types (in molecules as reactants), you end with those same 50 atoms (now in molecules as products), and since mass comes from atoms, the total mass stays constant. This is why balanced equations work—they ensure atom counts match on both sides, which guarantees mass conservation. However, in OPEN systems where gases can escape or be absorbed from the air, the MEASURED mass may appear to change even though total mass is actually conserved—you just have to account for gases that left or entered the system! In this sealed container, the reaction changes 3 gas molecules (2H2 + O2) into 2 liquid molecules, but the total mass remains 80 g because the same atoms (4 H and 2 O) are present before and after, just rearranged. Choice B correctly applies conservation of mass by recognizing that mass depends on atoms, not the number of molecules, so it stays constant despite the molecule count change. Choice A fails because it supports the student's incorrect claim that fewer molecules mean less mass, but conservation is about atoms, and the sealed system shows no mass change. Using conservation of mass: (1) In CLOSED systems (sealed container, nothing escapes): total mass before = total mass after, always! Add all reactant masses, add all product masses, they should match exactly. If they don't in data, measurement error occurred. (2) In OPEN systems (reaction in open air, unsealed): APPARENT mass may change because gases escape or enter. Mass INCREASE: gas from air absorbed (oxygen combining with substance during burning, rusting). Mass DECREASE: gas released to air (CO2, H2O vapor from combustion escaping). True total mass (including gases) still conserved, but you need to account for the gas! (3) To verify conservation: list ALL substances including gases. Example: burning 10g wood in open air leaves 1g ash—where did 9g go? Answer: 9g became CO2 and H2O vapor (gases escaped). Total: 10g wood + oxygen from air → 1g ash + 9g gases. Mass conserved when all counted! The "why mass appears to change" explanation: (1) Identify if system is open or closed. (2) If closed and mass changes in data, error occurred (conservation violated only by measurement mistakes). (3) If open and mass increases, look for gas absorption (combining with oxygen from air is most common). (4) If open and mass decreases, look for gas release (CO2, H2O vapor, or other gases escaping). (5) Explain: "Mass appears to decrease but is actually conserved because [specific gas] escaped; if measured in closed system, that gas would be captured and total mass would be constant." This accounting explains apparent violations while affirming conservation! Wonderful insight on atoms vs. molecules—keep thinking at that level!
15. Calcium carbonate decomposes when heated: $\text{CaCO}_3(s) \rightarrow \text{CaO}(s) + \text{CO}_2(g)$ A student heats 25 g of $\text{CaCO}_3$ in an open crucible. After heating, 14 g of solid $\text{CaO}$ remains in the crucible. Which statement best accounts for the missing mass?
    Answer: 11 g of $\text{CO}_2$ gas was produced and escaped into the air; mass would balance in a closed system. This question tests your understanding of the law of conservation of mass—the principle that mass is neither created nor destroyed in chemical reactions, only rearranged as atoms reorganize into different substances. The law of conservation of mass states that the total mass of all reactants must equal the total mass of all products because atoms are not created or destroyed in chemical reactions, merely rearranged: if you start with 50 atoms of various types (in molecules as reactants), you end with those same 50 atoms (now in molecules as products), and since mass comes from atoms, the total mass stays constant. In this decomposition reaction, we started with 25 g of CaCO₃ and ended with only 14 g of CaO in the crucible, meaning 11 g is missing—this is because CO₂ gas was produced and escaped into the air from the open crucible, taking its mass with it. Choice A correctly applies conservation of mass by recognizing that the 11 g difference equals the mass of CO₂ that escaped; if this were done in a closed system, we'd capture that CO₂ and find total mass = 25 g (14 g CaO + 11 g CO₂). Choice B incorrectly claims mass was destroyed (violating conservation), Choice C wrongly suggests solids lose mass just from heating without gas formation, and Choice D absurdly claims heat absorption reduces product mass. Using conservation of mass: In this open system, mass appears to decrease because CO₂ gas escapes. The calculation confirms conservation: 25 g CaCO₃ → 14 g CaO + 11 g CO₂ (escaped), with all calcium, carbon, and oxygen atoms accounted for, just redistributed between solid CaO and gaseous CO₂.
16. A reaction is carried out in a sealed container: A(s) + B(s) → C(s). The masses are: - A = 14 g - B = 5 g After the reaction, the mass of product C is measured. What mass of C should be produced (assuming all reactants turn into C and nothing escapes)?
    Answer: 19 g This question tests your understanding of the law of conservation of mass—the principle that mass is neither created nor destroyed in chemical reactions, only rearranged as atoms reorganize into different substances. The law of conservation of mass states that the total mass of all reactants must equal the total mass of all products because atoms are not created or destroyed in chemical reactions, merely rearranged: if you start with 50 atoms of various types (in molecules as reactants), you end with those same 50 atoms (now in molecules as products), and since mass comes from atoms, the total mass stays constant. This is why balanced equations work—they ensure atom counts match on both sides, which guarantees mass conservation. However, in OPEN systems where gases can escape or be absorbed from the air, the MEASURED mass may appear to change even though total mass is actually conserved—you just have to account for gases that left or entered the system! In this sealed container, all 14 g of A and 5 g of B react to form C with no escape, so the mass of C should be 14 g + 5 g = 19 g, conserving the total mass. Choice C correctly applies conservation of mass by recognizing that in a closed system, product mass equals reactant mass. Choice A fails because 9 g would imply mass loss, but in a sealed system with no gases mentioned, conservation requires the full sum; perhaps confusing subtraction instead of addition. Using conservation of mass: (1) In CLOSED systems (sealed container, nothing escapes): total mass before = total mass after, always! Add all reactant masses, add all product masses, they should match exactly. If they don't in data, measurement error occurred. (2) In OPEN systems (reaction in open air, unsealed): APPARENT mass may change because gases escape or enter. Mass INCREASE: gas from air absorbed (oxygen combining with substance during burning, rusting). Mass DECREASE: gas released to air (CO2, H2O vapor from combustion escaping). True total mass (including gases) still conserved, but you need to account for the gas! (3) To verify conservation: list ALL substances including gases. Example: burning 10g wood in open air leaves 1g ash—where did 9g go? Answer: 9g became CO2 and H2O vapor (gases escaped). Total: 10g wood + oxygen from air → 1g ash + 9g gases. Mass conserved when all counted! The "why mass appears to change" explanation: (1) Identify if system is open or closed. (2) If closed and mass changes in data, error occurred (conservation violated only by measurement mistakes). (3) If open and mass increases, look for gas absorption (combining with oxygen from air is most common). (4) If open and mass decreases, look for gas release (CO2, H2O vapor, or other gases escaping). (5) Explain: "Mass appears to decrease but is actually conserved because [specific gas] escaped; if measured in closed system, that gas would be captured and total mass would be constant." This accounting explains apparent violations while affirming conservation! Awesome addition skills—conservation is all about that total!
17. Baking soda and vinegar react in an open beaker: $\text{NaHCO}_3 + \text{HC}_2\text{H}_3\text{O}_2 \rightarrow \text{CO}_2 + \text{H}_2\text{O} + \text{NaC}_2\text{H}_3\text{O}_2$ Mass of beaker + reactants before mixing: 210 g Mass of beaker + remaining contents after bubbling stops: 206 g Which is the best explanation for the 4 g decrease?
    Answer: Some product, mainly $\text{CO}_2$ gas, escaped into the air; the total mass would be conserved if the system were closed. This question tests your understanding of the law of conservation of mass—the principle that mass is neither created nor destroyed in chemical reactions, only rearranged as atoms reorganize into different substances. The law of conservation of mass states that the total mass of all reactants must equal the total mass of all products because atoms are not created or destroyed in chemical reactions, merely rearranged: if you start with 50 atoms of various types (in molecules as reactants), you end with those same 50 atoms (now in molecules as products), and since mass comes from atoms, the total mass stays constant. In this open beaker experiment, the mass decreased from 210 g to 206 g (a 4 g loss) because CO₂ gas produced in the reaction escaped into the air—the bubbling you observe is CO₂ leaving the system, taking its mass with it. Choice B correctly applies conservation of mass by recognizing that in an open system, gases can escape, and if we could capture that escaped CO₂ (in a closed system), we'd find it has exactly 4 g of mass, making the total mass conserved. Choice A incorrectly claims mass was destroyed as heat (mass cannot be destroyed in chemical reactions), Choice C wrongly suggests the beaker absorbed oxygen to lower mass (absorbing would increase mass, not decrease it), and Choice D incorrectly links volume changes to mass changes. Using conservation of mass in open systems: When mass appears to decrease, look for gas release—here, CO₂ bubbles escaping explain the 4 g loss. The reaction still conserves mass: all the carbon and oxygen atoms that were in the reactants are still present, just some left as CO₂ gas into the air.
18. A student reacts 18 g of reactant mixture in an open container. After the reaction, only 11 g of solid product is collected in the container, and a gas was observed bubbling out during the reaction. Which statement best explains the results without violating the law of conservation of mass?
    Answer: The missing 7 g is most likely gaseous product that escaped; if all products (including gas) were captured, total mass would match the reactants. This question tests your understanding of the law of conservation of mass—the principle that mass is neither created nor destroyed in chemical reactions, only rearranged as atoms reorganize into different substances. The law of conservation of mass states that the total mass of all reactants must equal the total mass of all products because atoms are not created or destroyed in chemical reactions, merely rearranged: if you start with 50 atoms of various types (in molecules as reactants), you end with those same 50 atoms (now in molecules as products), and since mass comes from atoms, the total mass stays constant. This is why balanced equations work—they ensure atom counts match on both sides, which guarantees mass conservation. However, in OPEN systems where gases can escape or be absorbed from the air, the MEASURED mass may appear to change even though total mass is actually conserved—you just have to account for gases that left or entered the system! In this open container, 18 g of reactants produce 11 g of solid and 7 g of gas that escapes (as observed by bubbling), so the measured solid is less, but total mass including the gas would be 18 g if captured. Choice B correctly applies conservation of mass by recognizing that the missing mass is the escaped gaseous product, and capturing it would show conservation. Choice D fails because it suggests mass became energy, but conservation of mass holds in chemical reactions—energy changes don't destroy mass; the gas simply wasn't measured. Using conservation of mass: (1) In CLOSED systems (sealed container, nothing escapes): total mass before = total mass after, always! Add all reactant masses, add all product masses, they should match exactly. If they don't in data, measurement error occurred. (2) In OPEN systems (reaction in open air, unsealed): APPARENT mass may change because gases escape or enter. Mass INCREASE: gas from air absorbed (oxygen combining with substance during burning, rusting). Mass DECREASE: gas released to air (CO2, H2O vapor from combustion escaping). True total mass (including gases) still conserved, but you need to account for the gas! (3) To verify conservation: list ALL substances including gases. Example: burning 10g wood in open air leaves 1g ash—where did 9g go? Answer: 9g became CO2 and H2O vapor (gases escaped). Total: 10g wood + oxygen from air → 1g ash + 9g gases. Mass conserved when all counted! The "why mass appears to change" explanation: (1) Identify if system is open or closed. (2) If closed and mass changes in data, error occurred (conservation violated only by measurement mistakes). (3) If open and mass increases, look for gas absorption (combining with oxygen from air is most common). (4) If open and mass decreases, look for gas release (CO2, H2O vapor, or other gases escaping). (5) Explain: "Mass appears to decrease but is actually conserved because [specific gas] escaped; if measured in closed system, that gas would be captured and total mass would be constant." This accounting explains apparent violations while affirming conservation! Excellent— you've conserved mass like a pro in this scenario!
19. A reaction is performed, but the container is accidentally left unsealed: $\text{Zn}(s) + 2\text{HCl}(aq) \rightarrow \text{ZnCl}_2(aq) + \text{H}_2(g)$ Mass of zinc used: 5 g Mass of hydrochloric acid solution used: 20 g Total mass measured in the beaker before reaction: 150 g Total mass measured in the beaker after reaction: 148 g Which conclusion is most reasonable?
    Answer: The 2 g decrease is likely because $\text{H}_2$ gas escaped; if the gas were trapped, the total mass would stay the same. This question tests your understanding of the law of conservation of mass—the principle that mass is neither created nor destroyed in chemical reactions, only rearranged as atoms reorganize into different substances. The law of conservation of mass states that the total mass of all reactants must equal the total mass of all products because atoms are not created or destroyed in chemical reactions, merely rearranged: if you start with 50 atoms of various types (in molecules as reactants), you end with those same 50 atoms (now in molecules as products), and since mass comes from atoms, the total mass stays constant. In this unsealed container reaction, the mass decreased from 150 g to 148 g (a 2 g loss) because H₂ gas produced in the reaction escaped into the air—hydrogen gas is very light and readily escapes from open containers, taking its mass with it. Choice B correctly applies conservation of mass by recognizing that the 2 g decrease is due to H₂ gas escaping; if the gas were trapped in a sealed container, the total mass would remain 150 g. Choice A incorrectly claims conservation was violated (it wasn't—the H₂ just escaped), Choice C wrongly suggests fizzing makes liquids lighter, and Choice D incorrectly claims atoms converted to energy. Using conservation of mass in open systems: When zinc reacts with HCl, it produces H₂ gas which escapes if the container is open. The 2 g loss equals the mass of H₂ that escaped—mass is still conserved, but you must account for the gas that left the system. In a sealed container, that H₂ would be trapped and total mass would remain constant.
20. A student reacts 12 g of solution A with 8 g of solution B in a sealed plastic bag. A solid precipitate forms, but no material can enter or leave the bag. What should be the total mass of all products inside the bag after the reaction is complete?
    Answer: 20 g This question tests your understanding of the law of conservation of mass—the principle that mass is neither created nor destroyed in chemical reactions, only rearranged as atoms reorganize into different substances. The law of conservation of mass states that the total mass of all reactants must equal the total mass of all products because atoms are not created or destroyed in chemical reactions, merely rearranged: if you start with 50 atoms of various types (in molecules as reactants), you end with those same 50 atoms (now in molecules as products), and since mass comes from atoms, the total mass stays constant. This is why balanced equations work—they ensure atom counts match on both sides, which guarantees mass conservation. However, in OPEN systems where gases can escape or be absorbed from the air, the MEASURED mass may appear to change even though total mass is actually conserved—you just have to account for gases that left or entered the system! In this sealed bag, the total reactant mass is 12 g + 8 g = 20 g, and since nothing can enter or leave, the products (solid precipitate and any other substances) must total 20 g, with atoms rearranged but conserved. Choice A correctly applies conservation of mass by recognizing that in closed systems, total mass remains constant regardless of the reaction forming a precipitate. Choice D fails by claiming reactions use up mass, which contradicts the law since mass is conserved through atom preservation; choices B and C suggest incorrect totals without basis in the given masses or conservation principles. Using conservation of mass: (1) In CLOSED systems (sealed container, nothing escapes): total mass before = total mass after, always! Add all reactant masses, add all product masses, they should match exactly. If they don't in data, measurement error occurred. (2) In OPEN systems (reaction in open air, unsealed): APPARENT mass may change because gases escape or enter. Mass INCREASE: gas from air absorbed (oxygen combining with substance during burning, rusting). Mass DECREASE: gas released to air (CO2, H2O vapor from combustion escaping). True total mass (including gases) still conserved, but you need to account for the gas! (3) To verify conservation: list ALL substances including gases. Example: burning 10g wood in open air leaves 1g ash—where did 9g go? Answer: 9g became CO2 and H2O vapor (gases escaped). Total: 10g wood + oxygen from air → 1g ash + 9g gases. Mass conserved when all counted! The "why mass appears to change" explanation: (1) Identify if system is open or closed. (2) If closed and mass changes in data, error occurred (conservation violated only by measurement mistakes). (3) If open and mass increases, look for gas absorption (combining with oxygen from air is most common). (4) If open and mass decreases, look for gas release (CO2, H2O vapor, or other gases escaping). (5) Explain: "Mass appears to decrease but is actually conserved because [specific gas] escaped; if measured in closed system, that gas would be captured and total mass would be constant." This accounting explains apparent violations while affirming conservation!

Start this quiz