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.

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

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 HClO3.

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 HClO3. 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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