Oxidation Number Questions

The oxidation number is a theoretical concept. It tells about the hypothetical charge of an atom if all of its bonds are ionic. It tells us about the extent of oxidation of an atom in a molecule. It can be either positive, negative or neutral.

Oxidation Number Chemistry Questions with Solutions

Q1. What is the oxidation number of sulfur in sulphuric acid (H₂SO₄)?

1. 4
2. 6
3. 8
4. None of the above

Answer: (b), The oxidation number of sulphur in sulphuric acid (H₂SO₄) is 6.

Calculation:

1 X 2 + x + 4 X – 2 = 0

2 + x – 8 = 0

x = 6

So, option (b) is correct.

Q2. What is the oxidation number of chromium in calcium dichromate (CaCr₂O₇)?

1. 4
2. 6
3. 8
4. None of the above

Answer: (b), The oxidation number of chromium in calcium dichromate (CaCr₂O₇) is 6.

Calculation:

2 + 2x + 7 X (- 2) = 0

2 + 2x – 14 = 0

2x = 12

x = 6

So, option (b) is correct.

Q3. What is the oxidation number of nitrogen in nitric acid (HNO₃)?

1. 3
2. 5
3. 7
4. None of the above

Answer: (b), The oxidation number of nitrogen in nitric acid (HNO₃) is 5.

Calculation:

1 + x + 3 X ( -2) = 0

1 + x – 6 = 0

x – 5 = 0

x = 5

So, option (b) is correct.

Q4. Which of the following statements between HClO₄ and HClO₃ is true?

1. The oxidation number of chlorine in HClO₄ has been decreased in HClO₃
2. The oxidation numbers for all atoms are the same in both molecules
3. The oxidation number for chlorine in HClO₄ has increased in HClO₃
4. The oxidation number of oxygen in HClO₄ has been decreased in HClO₃

Answer: (a), The oxidation number of chlorine in HClO₄ has been decreased in HClO₃ is true.

Explanation: To understand it better, foremost we will calculate the oxidation number of HClO₄ and HClO_3.

The oxidation number of chlorine in HClO₄ will be:

1 + x + 4 X ( -2) = 0

1 + x – 8 = 0

x – 7 = 0

x = 7

The oxidation number of chlorine in HClO₃ will be:

1 + x + 3 X ( -2) = 0

1 + x – 6 = 0

x – 5 = 0

x = 5

Thus, we can clearly see that oxidation number in HClO₄ has been decreased in HClO_3. So option (a) is correct.

Q5. What is the oxidation number of carbon in CH₂Cl₂?

1. 2
2. 4
3. 6
4. None of the above.

Answer: (d), The oxidation number of carbon in CH₂Cl₂ is zero.

Calculation:

x + 2 X 1 + 2 X ( -1) = 0

x + 2 – 2 = 0

x = 0

The oxidation number of carbon in CH₂Cl₂ is zero.

So, option (d) is correct.

Q6. Chlorine is in +1 oxidation number in

1. HCl
2. HClO₄
3. ICl
4. Cl₂O
5. None of the above

Answer: (d), Chlorine is in +1 oxidation number in Cl₂O.

Explanation: To understand it better, foremost we will calculate the oxidation number of HCl, HClO₄, ICl and Cl₂O.

The oxidation number of chlorine in HCl will be:

1 + x = 0

x = -1

The oxidation number of chlorine in HClO₄ will be:

1 + x + 4 X ( -2) = 0

1 + x – 8 = 0

x – 7 = 0

x = 7

The oxidation number of chlorine in ICl will be:

1 + x = 0

x = -1

The oxidation number of chlorine in Cl₂O will be:

2x + ( -2) = 0

2x = 2

x = 1

Thus, chlorine is in +1 oxidation number in Cl₂O.

So, option (d) is correct.

Q7. When K₂Cr₂O₇ is converted to K₂CrO₄, the change in the oxidation number of chromium is

1. 0
2. 2
3. 4
4. None of the above

Answer: (a), When K₂Cr₂O₇ is converted to K₂CrO₄, the change in the oxidation number of chromium is zero.

Explanation: To understand it better, foremost we will calculate the oxidation of K₂Cr₂O₇ and K₂CrO₄.

The oxidation number of chromium in K₂Cr₂O₇ is:

2 X 1 + 2x + 7 X ( -2) = 0

2 + 2x – 14 = 0

2x – 12 = 0

2x = 12

x = 6

The oxidation number of chromium in K₂CrO₄ is:

2 X 1 + x + 4 X ( -2) = 0

2 + x – 8 = 0

x – 6 = 0

x = 6

Thus, the oxidation number of chromium does not change when K₂Cr₂O₇ is converted to K₂CrO₄.

So, option (a) is correct.

Q8. What is the oxidation number of chlorine in HOCl?

1. 1
2. 3
3. 5
4. None of the above

Answer: (a), The oxidation number of chlorine in HOCl is 1.

Calculation:

The oxidation number of chlorine in HOCl will be:

1 + ( -2) + x = 0

-1 + x = 0

x = 1

The oxidation number of chlorine in HOCl is one.

So, option (a) is correct.

Q9. Oxidation number of oxygen in O₂ molecule is

1. 0
2. 1
3. 2
4. None of the above

Answer: (a), Oxidation number of oxygen in O₂ molecule is zero.

Q10. The process in which oxidation number increases is known as

1. Oxidation
2. Reduction
3. Catalysis
4. None of the above

Answer: (a), The process in which oxidation number increases is known as oxidation.

Q11. Which element in the given compounds has the highest oxidation number?

1. Sulphur in SO₃
2. Carbon in CO₂
3. Aluminum in AlCl₃
4. Sulphur in CaS

Answer: (a) Sulphur in SO₃ will have the highest oxidation number.

Explanation: To understand it better, foremost we will calculate the oxidation number of SO₃, CO₂, AlCl₃ and CaS.

The oxidation number of sulphur in SO₃ is

x + 3 X (- 2) = 0

x – 6 = 0

x = 6

The oxidation number of carbon in CO₂ is

x + 2 X (- 2) = 0

x – 4 = 0

x = 4

The oxidation number of aluminum in AlCl₃ is

x + 3 X (- 1) = 0

x – 3 = 0

x = 3

The oxidation number of sulphur in CaS is

2 + x = 0

x = – 2

Thus, the oxidation number of sulphur in SO₃ is highest.

So, option (a) is correct.

Q12. Name an element that always shows a negative oxidation number.

Answer: Fluorine always shows a negative ( -1) oxidation number.

Q13. What is the oxidation state?

Answer: The oxidation state is the number of electrons that a specific atom can gain, lose or share with another atom. It explains the degree of oxidation of an atom in a molecule.

Q14. What is the oxidation number?

Answer: The oxidation number is the charge that a central metal atom will have even after all the ligands have been removed from that atom.

Q15. What are the rules for finding oxidation numbers?

Answer: The oxidation number is the charge that a central metal atom will have even after all the ligands have been removed from that atom.

Rules for finding the oxidation number:

Rule 1: An atom has a zero oxidation number in its elemental form.

Example: The oxidation number of chlorine in the Cl₂ molecule is zero.

Rule 2: The oxidation number of an ion is equivalent to its charge.

Example: The charge of chlorine ion is -1, so the oxidation number of chlorine ion will be -1.

Rule 3: The oxidation number of alkali metals is +1, and alkaline earth metal is +2.

Example: The oxidation number of sodium is +1, while the oxidation number of calcium is +2.

Rule 4: Hydrogen has two probable oxidation numbers, i.e. +1 and -1.

Example: The oxidation number of hydrogen in NaH is -1, while the oxidation number in HCl is +1.

Rule 5: Oxygen has three probable oxidation numbers: +2, -2 and -1.

Example: The oxidation number of oxygen in H₂O is -2, while the oxidation number in OF₂ is +2. In contrast, the oxidation number of oxygen is -1 in H₂O₂.

Rule 6: The oxidation number of fluorine in any compound is -1.

Example: The oxidation number of fluorine in HF is -1.

Rule 7: The oxidation number of halogen is typically equal to -1 except when bonded with oxygen or fluorine atom.

Example: The oxidation number of chlorine in HCl is -1, while the oxidation number of chlorine in HClO₄ is +7.

Rule 8: The sum of the oxidation numbers of neutral compounds equals zero.

Example: The oxidation number of chlorine is -1 in HCl, while the oxidation number of hydrogen is +1, and their sum is equal to zero.

Practise Questions on Oxidation Number

Q1. Fluorine always shows a -1 oxidation number. Why?

Q2. Differentiate between oxidation state and oxidation number.

Q3. What is the oxidation number of chlorine in the perchlorate ion?

1. 5
2. 7
3. 9
4. None of the above

Q4. What is the oxidation number of carbon in carbon suboxide (C₃O₂)?

1. 4 / 3
2. 2 / 3
3. 1 / 3
4. None of the above

Q5. When Chlorine gas reacts with hot and concentrated sodium hydroxide solution, the oxidation number of chlorine changes from

1. Zero to -1 and Zero to +3
2. Zero to +1 and Zero to -3
3. Zero to +1 and Zero to -5
4. Zero to -1 and Zero to +5

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