Oxidation-Reduction - MCAT Physical
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Which of the following is the oxidizing agent in the following equation?
Which of the following is the oxidizing agent in the following equation?
An oxidizing agent is one that oxidizes another element and, becomes reduced in the process. Oxidizing and reducing agents are found in the reactants of the equation, eliminating Al3+ and Cr(s) as answer choices. Cr3+ gains three electrons to become Cr(s), which has an oxidation number of zero. Cr3+ becomes reduced, while Al(s) is oxidized, therefore, Cr3+ is the oxidizing agent. Remember "OIL RIG": Oxidation Is Loss of electrons and Reduction Is Gain of electrons.
An oxidizing agent is one that oxidizes another element and, becomes reduced in the process. Oxidizing and reducing agents are found in the reactants of the equation, eliminating Al3+ and Cr(s) as answer choices. Cr3+ gains three electrons to become Cr(s), which has an oxidation number of zero. Cr3+ becomes reduced, while Al(s) is oxidized, therefore, Cr3+ is the oxidizing agent. Remember "OIL RIG": Oxidation Is Loss of electrons and Reduction Is Gain of electrons.
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In the above unbalanced redox reaction, which best describes the oxidation/reduction roles of iron and oxygen?
In the above unbalanced redox reaction, which best describes the oxidation/reduction roles of iron and oxygen?
There are a couple options to solve this problem. First, we could simply remember that anything which combines with oxygen in a redox reaction is oxidized, and the associated oxygen is reduced.
Another option is to think about the oxidation numbers of each element (an element whose oxidation number increases is oxidized, and one whose oxidation number decreases is reduced). If we choose this method, recall that free elements (Fe and O2 in this case) have oxidation number 0. Also, oxygen in compounds has oxidation number -2. Oxygen's oxidation number decreases from 0 to -2, meaning that oxygen is reduced. Since oxidation and reduction occur at the same time in different elements of the redox reaction, iron's oxidation number must increase, so iron is oxidized.
There are a couple options to solve this problem. First, we could simply remember that anything which combines with oxygen in a redox reaction is oxidized, and the associated oxygen is reduced.
Another option is to think about the oxidation numbers of each element (an element whose oxidation number increases is oxidized, and one whose oxidation number decreases is reduced). If we choose this method, recall that free elements (Fe and O2 in this case) have oxidation number 0. Also, oxygen in compounds has oxidation number -2. Oxygen's oxidation number decreases from 0 to -2, meaning that oxygen is reduced. Since oxidation and reduction occur at the same time in different elements of the redox reaction, iron's oxidation number must increase, so iron is oxidized.
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Consider the following combustion reaction.
Which of the following statements correctly describes carbon in the reaction?
Consider the following combustion reaction.
Which of the following statements correctly describes carbon in the reaction?
This problem requires us to determine the oxidation number of carbon as a reactant and as a product. The oxidation number of carbon as a reactant is –4, because it is attached to four hydrogens each with a charge of +1. The overall charge on the molecule is zero, the carbon must cancel out the charges contributed by hydrogen.
Carbon as a product has an oxidation number of +4, because it is attached to two oxygens, each with an oxidation number of –2. Again, we know that the molecule is neutral, and carbon must balance the charges from oxygen
Since the oxidation number of carbon went from –4 to +4, we conclude that carbon has been oxidized in the reaction. Any atom that loses electrons is oxidized, while any atom to gain electrons is reduced.
This problem requires us to determine the oxidation number of carbon as a reactant and as a product. The oxidation number of carbon as a reactant is –4, because it is attached to four hydrogens each with a charge of +1. The overall charge on the molecule is zero, the carbon must cancel out the charges contributed by hydrogen.
Carbon as a product has an oxidation number of +4, because it is attached to two oxygens, each with an oxidation number of –2. Again, we know that the molecule is neutral, and carbon must balance the charges from oxygen
Since the oxidation number of carbon went from –4 to +4, we conclude that carbon has been oxidized in the reaction. Any atom that loses electrons is oxidized, while any atom to gain electrons is reduced.
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Consider the following reaction.
Which of the following atoms is oxidized in the reaction?
Consider the following reaction.
Which of the following atoms is oxidized in the reaction?
The atom that is oxidized will have a higher oxidation number as a reactant than as a product. Since reactant phosphorus (P4) is in elemental form, it has an oxidation number of 0. As a product, phosphorus (PO43-) has an oxidation number of +5.
Total charge = (P) + 4(O) = –3
–3 = (P) + 4(–2) = (P) + (–8)
p = +5
Because it is now more positive, we say that the phosphorus has been oxidized.
The atom that is oxidized will have a higher oxidation number as a reactant than as a product. Since reactant phosphorus (P4) is in elemental form, it has an oxidation number of 0. As a product, phosphorus (PO43-) has an oxidation number of +5.
Total charge = (P) + 4(O) = –3
–3 = (P) + 4(–2) = (P) + (–8)
p = +5
Because it is now more positive, we say that the phosphorus has been oxidized.
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Consider the following reaction.
What is the oxidizing reagent in the reaction?
Consider the following reaction.
What is the oxidizing reagent in the reaction?
The oxidizing reagent is the reactant, not the atom, that is responsible for receiving electrons from another atom in order to oxidize it. The oxygen gas goes from having an oxidation number of 0 to –2. This means that oxygen gas is reduced, but because it oxidized another atom by taking its electrons, we call it the oxidizing reagent.
Reactant oxygen is elemental, and has oxidation number 0.
Product oxygen is in H2O, with net charge of 0.
Total charge = 2(H) + (O) = 0
2(+1) + (O) = 2 + (O) = 0
O = –2
Decrease in oxidation number indicates reduction, and also identifies the oxidizing agent.
The oxidizing reagent is the reactant, not the atom, that is responsible for receiving electrons from another atom in order to oxidize it. The oxygen gas goes from having an oxidation number of 0 to –2. This means that oxygen gas is reduced, but because it oxidized another atom by taking its electrons, we call it the oxidizing reagent.
Reactant oxygen is elemental, and has oxidation number 0.
Product oxygen is in H2O, with net charge of 0.
Total charge = 2(H) + (O) = 0
2(+1) + (O) = 2 + (O) = 0
O = –2
Decrease in oxidation number indicates reduction, and also identifies the oxidizing agent.
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Consider the following oxidation-reduction reaction.
Which of the following statements is true?
Consider the following oxidation-reduction reaction.
Which of the following statements is true?
In the reaction, solid potassium (K) is initially in elemental form, so it has an oxidation state of 0. The potassium ions (K+) have a charge of +1 in the product. Since the potassium has lost an electron, it has been oxidized.
Since each hydrogen in reactants (NH3) has an oxidation state of +1, and the hydrogen gas (H2) has a charge of 0 as a product, the hydrogen has been reduced. Since it was reduced, we conclude that NH3 is the oxidizing reagent for the reaction.
In the reaction, solid potassium (K) is initially in elemental form, so it has an oxidation state of 0. The potassium ions (K+) have a charge of +1 in the product. Since the potassium has lost an electron, it has been oxidized.
Since each hydrogen in reactants (NH3) has an oxidation state of +1, and the hydrogen gas (H2) has a charge of 0 as a product, the hydrogen has been reduced. Since it was reduced, we conclude that NH3 is the oxidizing reagent for the reaction.
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Which compound is oxidized in the following reaction?
Which compound is oxidized in the following reaction?
Oxidation is defined by a loss of electrons, generally resulting in a more positive charge on an atom. Reduction is defined by a gain of electrons, generally resulting in a more negative charge on an atom. To identify the compound being oxidized, we need to find which compound is becoming more positive.
Zinc is initially neutral, but gains a positive two charge as a product. Hydrogen initially has one positive charge, but becomes neutral in the product. The distinction becomes even clearer if we break the overall reaction into two half-reactions.
We can see that zinc is losing electrons and hydrogen is gaining electrons. Zinc is thus being oxidized.
Oxidation is defined by a loss of electrons, generally resulting in a more positive charge on an atom. Reduction is defined by a gain of electrons, generally resulting in a more negative charge on an atom. To identify the compound being oxidized, we need to find which compound is becoming more positive.
Zinc is initially neutral, but gains a positive two charge as a product. Hydrogen initially has one positive charge, but becomes neutral in the product. The distinction becomes even clearer if we break the overall reaction into two half-reactions.
We can see that zinc is losing electrons and hydrogen is gaining electrons. Zinc is thus being oxidized.
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Consider the combustion of cyclohexane in air at 298K to give gaseous carbon dioxide and liquid water, as shown in this reaction.
In this reaction, __________ is the oxidizing agent and __________ is the reducing agent.
Consider the combustion of cyclohexane in air at 298K to give gaseous carbon dioxide and liquid water, as shown in this reaction.
In this reaction, __________ is the oxidizing agent and __________ is the reducing agent.
In this reaction, the oxidation number of oxygen goes from 
, in diatomic gaseous oxygen, to 
, in both carbon dioxide and water. This indicates that it has gained electrons; a gain of electrons indicates that oxygen has been reduced. Since it is reduced, it is the oxidizing agent.
The oxidation number of carbon goes from 
in cyclohexane to 
in carbon dioxide. This indicates that it has lost electrons; a loss of electrons indicates that carbon has been oxidized. Since it is oxidized, cyclohexane is the reducing agent.
In this reaction, the oxidation number of oxygen goes from
The oxidation number of carbon goes from
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Which of the following is an incorrect example of a combustion reaction?
Which of the following is an incorrect example of a combustion reaction?
We often hear of combustion reactions producing carbon dioxide and water. This is true when the reactants contain all three elements (hydrogen, oxygen, and carbon) needed to make carbon dioxide and water. The trick here is to understand that although combustion is just an oxidation reaction (ie, creating more carbon-oxygen bonds), all reactions must still be balanced. One answer choice fails to balance hydrogen in the reaction.
Although we do see an oxidation reaction with the formation of carbon dioxide, the reaction is NOT balanced. This reaction cannot possibly take place as written.
We often hear of combustion reactions producing carbon dioxide and water. This is true when the reactants contain all three elements (hydrogen, oxygen, and carbon) needed to make carbon dioxide and water. The trick here is to understand that although combustion is just an oxidation reaction (ie, creating more carbon-oxygen bonds), all reactions must still be balanced. One answer choice fails to balance hydrogen in the reaction.
Although we do see an oxidation reaction with the formation of carbon dioxide, the reaction is NOT balanced. This reaction cannot possibly take place as written.
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The Haber-Bosch process, or simply the Haber process, is a common industrial reaction that generates ammonia from nitrogen and hydrogen gas. A worker in a company generates ammonia from the Haber process. He then dissociates the gaseous ammonia in water to produce an aqueous solution. Since ammonia is a base, it will accept a proton from water, generating 
and ammonium ion products. The two reactions involved are:
A redox (reduction and oxidation) reaction involves a change in the oxidation state of atoms. Oxidation of an atom involves an increase in the oxidation state, whereas reduction involves a decrease in the oxidation state. A oxidized atom will usually become __________.
The Haber-Bosch process, or simply the Haber process, is a common industrial reaction that generates ammonia from nitrogen and hydrogen gas. A worker in a company generates ammonia from the Haber process. He then dissociates the gaseous ammonia in water to produce an aqueous solution. Since ammonia is a base, it will accept a proton from water, generating
A redox (reduction and oxidation) reaction involves a change in the oxidation state of atoms. Oxidation of an atom involves an increase in the oxidation state, whereas reduction involves a decrease in the oxidation state. A oxidized atom will usually become __________.
The question states that an oxidized atom will increase its oxidation state, which means that the atom will lose electrons and become more positively charged. Since the atom will lose electrons, the number of electrons will be less than the number of protons. Electrons are negatively charged and protons are positively charged; therefore, the presence of more protons in an oxidized atom will make the atom more positive. An oxidized atom will usually become a cation because its number of electrons will decrease.
Reduced atoms will gain electrons and become more negatively charged (decrease oxidation state). Notice that the reactions given in the passage are not redox reactions because the atoms in both reactions retain their oxidation states.
The question states that an oxidized atom will increase its oxidation state, which means that the atom will lose electrons and become more positively charged. Since the atom will lose electrons, the number of electrons will be less than the number of protons. Electrons are negatively charged and protons are positively charged; therefore, the presence of more protons in an oxidized atom will make the atom more positive. An oxidized atom will usually become a cation because its number of electrons will decrease.
Reduced atoms will gain electrons and become more negatively charged (decrease oxidation state). Notice that the reactions given in the passage are not redox reactions because the atoms in both reactions retain their oxidation states.
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Consider the redox reaction:
What is the balanced equation for the above redox reaction in acidic solution?
Consider the redox reaction:
What is the balanced equation for the above redox reaction in acidic solution?
1. Determine the oxidation number of each element.
Reactants: 
Products: 
2. Write and balance the half reactions.
Oxidation (loss of electrons): 
Reduction (gain of electrons): 
3. Balance oxygen by adding water (from the solution) into the half reactions.
Oxidation: 
Reduction: 
4. Balance hydrogen by adding 
ions from the acid into the half reactions.
Oxidation: 
Reduction: 
5. Multiply each half reaction times an integer such that the electrons cancel when the equations are added. The oxidation reaction will be multiplied by five, and the reduction reaction will be multiplied by two.
Oxidation times 5: 
Reduction times 2: 
6. Add the half reactions together. The electrons will cancel from both the reactants and products.
1. Determine the oxidation number of each element.
Reactants:
Products:
2. Write and balance the half reactions.
Oxidation (loss of electrons):
Reduction (gain of electrons):
3. Balance oxygen by adding water (from the solution) into the half reactions.
Oxidation:
Reduction:
4. Balance hydrogen by adding
Oxidation:
Reduction:
5. Multiply each half reaction times an integer such that the electrons cancel when the equations are added. The oxidation reaction will be multiplied by five, and the reduction reaction will be multiplied by two.
Oxidation times 5:
Reduction times 2:
6. Add the half reactions together. The electrons will cancel from both the reactants and products.
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What is the oxidation state of sulfur in H2SO4?
What is the oxidation state of sulfur in H2SO4?
Since there is no overall charge on the compound the oxidation states must cancel out. Although the oxidation state of hydrides are +1, there are two in the compound that must be accounted for. The same is true for oxygen; although the oxidation number of oxygen is -2, there are four oxygens present accounting for a total of -8. The +2 state contribution from the hydrides and -8 from the oxygens results in a -6 charge. The oxidation state on sulfur must be +6 for the molecule to be neutral.
Since there is no overall charge on the compound the oxidation states must cancel out. Although the oxidation state of hydrides are +1, there are two in the compound that must be accounted for. The same is true for oxygen; although the oxidation number of oxygen is -2, there are four oxygens present accounting for a total of -8. The +2 state contribution from the hydrides and -8 from the oxygens results in a -6 charge. The oxidation state on sulfur must be +6 for the molecule to be neutral.
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What is the oxidation number of chromium in calcium dichromate (CaCr2O7)?
What is the oxidation number of chromium in calcium dichromate (CaCr2O7)?
Since the calcium ion has a charge of +2, the dichromate anion must have a charge of -2. Oxygen always has an oxidation number of -2, so the seven oxygens in the molecule have a total number of -14. That leaves a charge of +12 that the two chromiums must balance out (-2 - (-14) = 12). Each chromium must therefore have an oxidation number of +6.
Since the calcium ion has a charge of +2, the dichromate anion must have a charge of -2. Oxygen always has an oxidation number of -2, so the seven oxygens in the molecule have a total number of -14. That leaves a charge of +12 that the two chromiums must balance out (-2 - (-14) = 12). Each chromium must therefore have an oxidation number of +6.
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What is the oxidation state of nitrogen in HNO3?
What is the oxidation state of nitrogen in HNO3?
When applying oxidation numbers there are certain hierarchical rules that must be followed.
1. The sum of oxidation states of all the elements in a molecule must add up to the overall charge.
2. Group 1 and Group 2 elements have +1 and +2 oxidation states, respectively.
3. Fluorine has an oxidation state of –1.
4. Hydrogen has an oxidation state of +1 (except in metal hydrides).
5. Oxygen has an oxidation state of –2.
6. Elements of the same group (excluding transition metals) generally have the same oxidation state.
When applying the oxidation state to HNO3, hydrogen has a +1 oxidation state, and each of the oxygen molecules has a –2 oxidation state. Since there is only one hydrogen molecule and three oxygen molecules, the oxidation state of nitrogen must balance out the charge of the hydrogen molecule and oxygen molecules combined.
Nitrogen must have a +5 oxidation state.
When applying oxidation numbers there are certain hierarchical rules that must be followed.
1. The sum of oxidation states of all the elements in a molecule must add up to the overall charge.
2. Group 1 and Group 2 elements have +1 and +2 oxidation states, respectively.
3. Fluorine has an oxidation state of –1.
4. Hydrogen has an oxidation state of +1 (except in metal hydrides).
5. Oxygen has an oxidation state of –2.
6. Elements of the same group (excluding transition metals) generally have the same oxidation state.
When applying the oxidation state to HNO3, hydrogen has a +1 oxidation state, and each of the oxygen molecules has a –2 oxidation state. Since there is only one hydrogen molecule and three oxygen molecules, the oxidation state of nitrogen must balance out the charge of the hydrogen molecule and oxygen molecules combined.
Nitrogen must have a +5 oxidation state.
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What is the oxidation number of hydrogen in NaH?
What is the oxidation number of hydrogen in NaH?
When applying oxidation numbers there are certain hierarchical rules that must be followed.
1. The sum of oxidation states of all the elements in a molecule must add up to the overall charge.
2. Group 1 and Group 2 elements have +1 and +2 oxidation states, respectively.
3. Fluorine has an oxidation state of –1.
4. Hydrogen has an oxidation state of +1 (except in metal hydrides).
5. Oxygen has an oxidation state of –2.
6. Elements of the same group (excluding transition metals) generally have the same oxidation state.
Following the oxidation rules, group 1 elements must have an oxidation number of +1.
To balance out the overall charge of zero, the hydrogen in this compound must have an oxidation state of –1. Note that NaH is a metal hydride; therefore, the oxidation state of hydrogen is not necessarily +1.
When applying oxidation numbers there are certain hierarchical rules that must be followed.
1. The sum of oxidation states of all the elements in a molecule must add up to the overall charge.
2. Group 1 and Group 2 elements have +1 and +2 oxidation states, respectively.
3. Fluorine has an oxidation state of –1.
4. Hydrogen has an oxidation state of +1 (except in metal hydrides).
5. Oxygen has an oxidation state of –2.
6. Elements of the same group (excluding transition metals) generally have the same oxidation state.
Following the oxidation rules, group 1 elements must have an oxidation number of +1.
To balance out the overall charge of zero, the hydrogen in this compound must have an oxidation state of –1. Note that NaH is a metal hydride; therefore, the oxidation state of hydrogen is not necessarily +1.
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What is the correct oxidation number of zinc in the compound 
?
What is the correct oxidation number of zinc in the compound
The sum of oxidation numbers of each element in this compound must add to the total charge of -2. Hydrogen always has an oxidation number of +1, and oxygen always has an oxidation number of -2.
So, in total for oxygen we have (4) * (-2) = -8, and for hydrogen we have (4) * (+1) = +4.
Combining the oxygen and hydrogen, (+4) + (-8) = -4, so we need an additional +2 to achieve the total charge of -2. Zn must provide this balancing charge, and have an oxidation number of +2.
The sum of oxidation numbers of each element in this compound must add to the total charge of -2. Hydrogen always has an oxidation number of +1, and oxygen always has an oxidation number of -2.
So, in total for oxygen we have (4) * (-2) = -8, and for hydrogen we have (4) * (+1) = +4.
Combining the oxygen and hydrogen, (+4) + (-8) = -4, so we need an additional +2 to achieve the total charge of -2. Zn must provide this balancing charge, and have an oxidation number of +2.
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Which molecule contains an atom with an oxidation state of 
?
Which molecule contains an atom with an oxidation state of
The rules for oxidation state note that certain atoms will have constant oxidation values, while others are more likely to vary. Oxygen, for example, will always have an oxidation state of 
, alkali metals will always have an oxidation state of 
, and hydrogen will (almost) always have an oxidation state of 
.
All of the given compounds are neutral, meaning that their oxidation numbers must sum up to zero. Using our listed rules to look at these molecules, we can try to find any missing values.
Chromium in potassium dichromate is the only atom in this set to have an oxidation state of 
.
The rules for oxidation state note that certain atoms will have constant oxidation values, while others are more likely to vary. Oxygen, for example, will always have an oxidation state of
All of the given compounds are neutral, meaning that their oxidation numbers must sum up to zero. Using our listed rules to look at these molecules, we can try to find any missing values.
Chromium in potassium dichromate is the only atom in this set to have an oxidation state of
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Which of the following statements between HClO4 and HClO3 is true?
Which of the following statements between HClO4 and HClO3 is true?
Oxidation number is a concept that you will also encounter in electrochemistry lectures. It is used to track electrons in an oxidation-reduction reaction. Overall, the sum of the oxidation states of all of the atoms in the molecule must equal the overall charge on the molecule. It is important to remember some of the commonly accepted oxidation states for atoms, so that you can predict the oxidation numbers on other atoms. Here are some key oxidation numbers useful to know for the MCAT:
1. Atoms in elemental form have an oxidation state of 0.
2. Halogens are commonly given an oxidation state of –1.
3. Hydrogen and alkali metals are commonly given an oxidation state of +1.
4. Oxygen is commonly given an oxidation number of –2.
Keeping these rules in mind, we can assume that the hydrogens and oxygens in the two compounds maintain the same oxidation state. Chlorine, however, is attached to one less oxygen in HClO3. As a result, the oxidation state of Cl has decreased from +7 to +5, when comparing HClO4 to HClO3.
Oxidation number is a concept that you will also encounter in electrochemistry lectures. It is used to track electrons in an oxidation-reduction reaction. Overall, the sum of the oxidation states of all of the atoms in the molecule must equal the overall charge on the molecule. It is important to remember some of the commonly accepted oxidation states for atoms, so that you can predict the oxidation numbers on other atoms. Here are some key oxidation numbers useful to know for the MCAT:
1. Atoms in elemental form have an oxidation state of 0.
2. Halogens are commonly given an oxidation state of –1.
3. Hydrogen and alkali metals are commonly given an oxidation state of +1.
4. Oxygen is commonly given an oxidation number of –2.
Keeping these rules in mind, we can assume that the hydrogens and oxygens in the two compounds maintain the same oxidation state. Chlorine, however, is attached to one less oxygen in HClO3. As a result, the oxidation state of Cl has decreased from +7 to +5, when comparing HClO4 to HClO3.
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Which element in the given compounds has the highest oxidation number?
Which element in the given compounds has the highest oxidation number?
We can calculate the oxidation number in each answer to find the greatest value.
In 
, oxygen will have a value of 
and the molecule is neutral. Sulfur must have an oxidation number of 
to balance the three oxygens.
In 
, oxygen will have a value of 
and the molecule is neutral. Carbon must have an oxidation number of 
to balance the two oxygens.
In 
, chlorine will have a value of 
and the molecule is neutral. Aluminum must have an oxidation number of 
to balance the three chlorines.
In 
, calcium will have a value of 
and the molecule is neutral. Sulfur must have an oxidation number of 
to balance the calcium.
is a molecule comprised of a single element; each atom will have an oxidation number of 
.
We can calculate the oxidation number in each answer to find the greatest value.
In
In
In
In
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What is the oxidation number of chlorine in the perchlorate ion?
What is the oxidation number of chlorine in the perchlorate ion?
To find the oxidation number, first remember that the total charge must be preserved. The total charge of the perchlorate ion is 
.
Each oxygen will have an oxidation number of 
. The oxidation of chlorine must be used to balance this negative charge.
To find the oxidation number, first remember that the total charge must be preserved. The total charge of the perchlorate ion is
Each oxygen will have an oxidation number of
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