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Chemistry Help: Explain Conservation Of Mass

Review real example questions for Explain Conservation Of Mass in Chemistry.

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A student mixes two solutions in a sealed jar. Reaction: $\mathrm{AgNO_3(aq) + NaCl(aq) \rightarrow AgCl(s) + NaNO_3(aq)}$ . Masses of reactant solutions added:

$\mathrm{AgNO_3}$ solution: 40 g
$\mathrm{NaCl}$ solution: 40 g The jar is sealed before mixing and remains sealed. After the precipitate forms, which total product mass is expected (not including the jar)?

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Question 1

A student mixes two solutions in a sealed jar. Reaction: $\mathrm{AgNO_3(aq) + NaCl(aq) \rightarrow AgCl(s) + NaNO_3(aq)}$ . Masses of reactant solutions added:

$\mathrm{AgNO_3}$ solution: 40 g
$\mathrm{NaCl}$ solution: 40 g The jar is sealed before mixing and remains sealed. After the precipitate forms, which total product mass is expected (not including the jar)?

80 g (correct answer)
40 g, because half the mass turns into a solid.
Less than 80 g, because ions combine and reduce mass.
More than 80 g, because a new compound formed.

Explanation: 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 jar (closed system), the total reactant mass is $40 \, \mathrm{g} \, \mathrm{AgNO_3} \, \text{solution} + 40 \, \mathrm{g} \, \mathrm{NaCl} \, \text{solution} = 80 \, \mathrm{g}$ , and after the precipitate forms, the total product mass (AgCl solid + NaNO3 solution) should remain 80 g since nothing escapes. Choice A correctly applies conservation of mass by recognizing that in a closed system, total mass is conserved at 80 g. For example, choice D fails because it suggests mass increases when a new compound forms, but conservation means mass stays the same regardless of new compounds. 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!

Question 2

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?

In Trial 1, atoms were destroyed; in Trial 2, atoms were conserved.
In Trial 1, CO $_2$ escaped to the air; in Trial 2, CO $_2$ was trapped, so the measured mass stayed constant. (correct answer)
In Trial 2, the balance reads higher because sealed flasks always weigh more than open cups.
In Trial 1, the reaction made less product; in Trial 2, it made more product because it was sealed.

Explanation: 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!

Question 3

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?

The student is correct: fewer gas molecules means less mass, but the container hides the change.
Mass is conserved because the same numbers of H and O atoms are present before and after; they are just rearranged into different molecules. (correct answer)
Mass is conserved only when the number of molecules stays the same, which happens here by coincidence.
Mass is conserved because gases have no mass, so only the liquid matters.

Explanation: 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!

Question 4

A student reacts two substances in an open container and records these masses:

Before reaction: Reactant X = 14 g, Reactant Y = 6 g (total = 20 g) After reaction: Product mixture in container = 17 g

Which is the most reasonable explanation for the "missing" 3 g?

3 g of mass was destroyed because products are more stable than reactants.
3 g of atoms disappeared during the reaction.
About 3 g of gaseous product likely escaped to the air, so it was not included in the final mass measurement. (correct answer)
The final mass must be 20 g because mass always appears constant, even in open systems.

Explanation: 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, the mass drops from 20 g to 17 g likely because about 3 g of gaseous product escaped, but if included, total mass would conserve at 20 g. Choice C correctly applies conservation of mass by recognizing that in open systems, apparent missing mass is often due to escaped gases not measured, but the law holds when accounting for them. Choice B fails by claiming atoms disappeared, which violates atom conservation; choice A suggests mass destruction due to stability, but mass is independent of stability, and choice D incorrectly states mass always appears constant even in open systems, ignoring gas effects. 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!

Question 5

In a sealed plastic bag, a student mixes 10 g of calcium chloride with 20 g of sodium carbonate solution. Reaction: $\mathrm{CaCl_2(aq) + Na_2CO_3(aq) \rightarrow CaCO_3(s) + 2NaCl(aq)}$ . The total mass of the sealed bag and contents is 85 g before mixing. After the solid forms, what should the total mass of the sealed bag and contents be?

65 g, because a solid formed and sank.
85 g, because mass is conserved in a closed (sealed) system. (correct answer)
Greater than 85 g, because new matter was created.
Less than 85 g, because the reactants were used up.

Explanation: 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 (a closed system), the total mass before mixing is 85 g (10 g CaCl2 + 20 g Na2CO3 solution + bag mass implied in total), and since nothing can escape or enter, the total mass after the reaction, including the solid CaCO3 and NaCl solution, should still be 85 g. Choice B correctly applies conservation of mass by recognizing that in a closed system, total mass is conserved at 85 g. For example, choice C fails because it suggests new matter was created, violating conservation, but mass cannot be created in chemical 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!

Question 6

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 $\mathrm{MCl_2}$ ?

15 g, because the gas disappears.
10 g, because only chlorine becomes part of the product.
25 g, because the masses of reactants add up and matter is conserved in a closed system. (correct answer)
Less than 25 g, because forming a compound reduces total mass.

Explanation: 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!

Question 7

A student mixes baking soda and vinegar in an open beaker: $\text{NaHCO}_3 + \text{HC}_2\text{H}_3\text{O}_2 \rightarrow \text{CO}_2(g) + \text{H}_2\text{O}(l) + \text{NaC}_2\text{H}_3\text{O}_2(aq)$ . Before mixing, the beaker + contents has a mass of 120 g. After the bubbling stops, the beaker + remaining contents has a mass of 116 g. Which statement best explains the 4 g mass decrease?

The reaction destroyed 4 g of matter as chemical bonds broke.
The mass decreased because $\text{CO}_2(g)$ produced in the reaction escaped into the air in the open system. (correct answer)
The mass decreased because liquids always have less mass after reacting.
The mass decreased because volume is not conserved, so mass is not conserved either.

Explanation: 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 beaker setup, the initial mass is 120 g, and after the reaction, it's 116 g, but the 4 g difference is due to CO2 gas escaping into the air, so the total mass including the escaped gas would still equal 120 g if captured. Choice B correctly applies conservation of mass by recognizing that in open systems, apparent mass decreases occur when gases like CO2 escape, but true mass is conserved when accounting for them. Choice A fails by incorrectly claiming matter is destroyed, which violates the law since atoms are only rearranged, not eliminated; similarly, choices C and D overlook the role of gases and misattribute the change to liquids or volume, but mass conservation holds independently of state or volume changes. 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!

Question 8

A student compares the same gas-producing reaction in two setups:

Setup 1 (open cup): Total mass before = 90 g; total mass after = 86 g. Setup 2 (sealed bag): Total mass before = 90 g; total mass after = 90 g.

Which statement best explains the difference?

Conservation of mass only applies in sealed bags, not in open cups.
The open cup lost mass because a gaseous product escaped; the sealed bag kept all products, so total mass stayed the same. (correct answer)
The sealed bag gained mass, but the scale could not detect it.
The open cup lost mass because the reaction used up atoms, while the sealed bag did not.

Explanation: 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 the open cup, mass drops from 90 g to 86 g because gaseous products escape, while in the sealed bag, mass stays at 90 g as all products, including gases, are retained, demonstrating conservation in closed systems. Choice B correctly applies conservation of mass by recognizing the difference between open and closed systems, where gases are accounted for in the sealed setup but lost in the open one. Choice A fails by limiting conservation to sealed systems only, but the law always holds, with apparent changes in open systems explained by gas movement; choice D incorrectly claims atoms are used up in open systems, violating the principle of atom preservation. 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!

Question 9

A reaction is performed in an open flask: $\mathrm{X(aq) \rightarrow Y(aq) + Z(g)}$ . The flask and solution have a mass of 200 g before the reaction. After the reaction, the flask and remaining liquid have a mass of 193 g. Which conclusion is most consistent with conservation of mass?

7 g of gas $\mathrm{Z}$ escaped to the air, so the mass of the measured system decreased. (correct answer)
7 g of mass was destroyed because gas formed.
The balance must be wrong because mass can never change, even in open systems.
The mass decreased because the number of molecules decreased, and molecule count is what is conserved.

Explanation: 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 flask, the reaction produces gas Z, which escapes, causing the measured mass to decrease from 200 g to 193 g, meaning 7 g of gas left the system, but total mass including the escaped gas would be conserved at 200 g. Choice A correctly applies conservation of mass by recognizing that in an open system, the gas escaped, leading to the apparent decrease. For example, choice B fails because it claims mass was destroyed, but conservation affirms no mass is destroyed; the gas simply left the measured 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 (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!

Question 10

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?

Mass was destroyed during the reaction because gas is lighter than solids and liquids.
The law of conservation of mass does not apply to reactions that produce bubbles.
CO $_2$ (g) escaped from the open beaker, so the measured mass decreased even though total mass would be conserved in a closed system. (correct answer)
The mass decreased because the volume decreased as the reactants were used up.

Explanation: 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!