Given below are two statements: Statement I: The oxidation number of oxygen in OF₂ is −2. Statement II: The oxidation number of hydrogen in NaH is +1. Choose the correct answer.

Both Statement I and Statement II are incorrect

Both Statement I and Statement II are correct

Statement I is correct but Statement II is incorrect

Statement I is incorrect but Statement II is correct

Redox Reactions

23 NEET-UG practice questions on Redox Reactions, part of Chemistry. The first 23 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

Question 1 · Medium
Given below are two statements: Statement I: The oxidation number of oxygen in OF₂ is −2. Statement II: The oxidation number of hydrogen in NaH is +1. Choose the correct answer.
1. ABoth Statement I and Statement II are correct
2. BBoth Statement I and Statement II are incorrect
3. CStatement I is correct but Statement II is incorrect
4. DStatement I is incorrect but Statement II is correct
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Answer: B · Both Statement I and Statement II are incorrect
Fluorine is always −1, so oxygen in OF₂ is +2. In metal hydrides, hydrogen is −1, as sodium is +1. Both statements are therefore false.
- A: Both statements are false, as both use the usual values wrongly.
- C: Statement I is false, since F forces oxygen to +2.
- D: Statement II is false, since hydrogen is −1 with a metal.
Remember this
Oxygen is positive in fluorides, and hydrogen is −1 in metal hydrides.
Question 2 · Hard
Exactly 1.26 g of H₂C₂O₄·2H₂O is dissolved to make 250 mL of solution. A 25 mL portion is acidified and titrated with 0.02 M KMnO₄: 2MnO₄⁻ + 5C₂O₄²⁻ + 16H⁺ → 2Mn²⁺ + 10CO₂ + 8H₂O What volume of KMnO₄ is needed? (H₂C₂O₄·2H₂O = 126, H₂C₂O₄ = 90)
1. A28 mL
2. B125 mL
3. C20 mL
4. D50 mL
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Answer: C · 20 mL
Oxalic acid = 1.26/126 = 0.01 mol in 250 mL, so 25 mL holds 1 mmol. KMnO₄ needed = 1 × 2/5 = 0.4 mmol, and the volume = 0.4/0.02 = 20 mL. Check by equivalents: 1 mmol × 2 = 2 meq, and V × 0.02 × 5 = 2 gives V = 20 mL.
- A: This uses the anhydrous molar mass for hydrated crystals.
- B: This inverts the ratio, using 5/2 instead of 2/5.
- D: This takes a 1 : 1 mole ratio.
Remember this
In acid, 2 mol KMnO₄ react with 5 mol oxalate; use the hydrate's molar mass for the weighed crystals.
Question 3 · Easy
In the reaction Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s), which species acts as the oxidising agent?
1. ACu²⁺
2. BZn
3. CCu
4. DZn²⁺
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Answer: A · Cu²⁺
Cu²⁺ gains two electrons and is reduced to Cu, while it oxidises Zn to Zn²⁺. The species that is reduced is the oxidising agent.
- B: Zn loses electrons, so it is the reducing agent.
- C: Cu is a product, formed by reduction of Cu²⁺.
- D: Zn²⁺ is a product, formed by oxidation of Zn.
Remember this
The oxidising agent is reduced; the reducing agent is oxidised.
Question 4 · Medium
An acidified permanganate solution oxidises iron(II): a MnO₄⁻ + b Fe²⁺ + c H⁺ → Mn²⁺ + Fe³⁺ + H₂O (not balanced) For the balanced ionic equation in lowest whole numbers, what are a, b and c?
1. A1, 3, 8
2. B1, 5, 4
3. C1, 5, 8
4. D2, 5, 16
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Answer: C · 1, 5, 8
Mn goes from +7 to +2, gaining 5 electrons, and each Fe²⁺ loses 1, so b = 5 per MnO₄⁻. Four O need 8 H⁺ to form 4 H₂O. The charges match: −1 + 10 + 8 = +17 and +2 + 15 = +17.
- A: This takes Mn as changing by three units, as it does in going to MnO₂.
- B: This gives H⁺ only half of what the four oxygen atoms need.
- D: This doubles MnO₄⁻ and H⁺ but not Fe²⁺.
Remember this
Balance electrons first, then O with H₂O, then H with H⁺, and confirm by charge.
Question 5 · Easy
What is the oxidation number of phosphorus in H₃PO₂?
1. A−1
2. B+5
3. C+3
4. D+1
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Answer: D · +1
With H = +1 and O = −2, the sum is 3(+1) + x + 2(−2) = 0, so x = +1. Only the sum over all atoms matters, so count each atom in the formula.
- A: This has the sign reversed.
- B: This is the value in H₃PO₄.
- C: This is the value in H₃PO₃.
Remember this
The oxidation numbers in a neutral molecule add up to zero.
Question 6 · Medium
Disproportionation occurs in which of the following reactions?
1. ACaCO₃(s) → CaO(s) + CO₂(g)
2. B2H₂O₂(aq) → 2H₂O(l) + O₂(g)
3. C2Mg(s) + O₂(g) → 2MgO(s)
4. D2KClO₃(s) → 2KCl(s) + 3O₂(g)
Answer and explanation in LearnFree account
Question 7 · Medium
Arrange the following in increasing order of the oxidation number of nitrogen. A. NO₂ B. HNO₃ C. N₂O D. NH₃
1. AD, A, C, B
2. BD, C, A, B
3. CB, A, C, D
4. DC, D, A, B
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Question 8 · Easy
In oxygen difluoride, OF₂, the oxidation number of oxygen is
1. A−2
2. B+2
3. C−1
4. D+1
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Question 9 · Hard
In a neutral or weakly basic solution, permanganate oxidises sulphite to sulphate and is itself reduced to MnO₂. When the ionic equation a MnO₄⁻ + b SO₃²⁻ + c H₂O → d MnO₂ + e SO₄²⁻ + f OH⁻ is balanced with the smallest whole numbers, what are a, b and f?
1. A2, 5, 2
2. B3, 2, 2
3. C2, 3, 2
4. D2, 3, 4
Answer and explanation in LearnFree account
Question 10 · Medium
Acidified potassium dichromate oxidises Fe²⁺ as follows: Cr₂O₇²⁻ + 6Fe²⁺ + 14H⁺ → 2Cr³⁺ + 6Fe³⁺ + 7H₂O What volume of 0.10 M K₂Cr₂O₇ is needed to oxidise all the Fe²⁺ in 30 mL of 0.20 M FeSO₄ solution?
1. A60 mL
2. B20 mL
3. C360 mL
4. D10 mL
Answer and explanation in LearnFree account
Question 11 · Medium
Match List-I with List-II. List-I (element in compound) A. Mn in KMnO₄ B. Cr in K₂Cr₂O₇ C. S in Na₂S₂O₃ D. Fe in Fe₃O₄ List-II (oxidation number, average where needed) I. +6 II. +8/3 III. +2 IV. +7 Choose the correct answer.
1. AA-IV, B-III, C-I, D-II
2. BA-IV, B-I, C-II, D-III
3. CA-I, B-IV, C-III, D-II
4. DA-IV, B-I, C-III, D-II
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Question 12 · Easy
In the reaction 2S₂O₃²⁻ + I₂ → S₄O₆²⁻ + 2I⁻, the average oxidation number of sulphur changes from
1. A+4 to +2.5
2. B+2 to +2.5
3. C+2.5 to +2
4. D+2 to +5
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Question 13 · Hard
Acidified KMnO₄ oxidises iron(II) oxalate, FeC₂O₄, completely: Fe²⁺ goes to Fe³⁺ and C₂O₄²⁻ goes to CO₂, while MnO₄⁻ is reduced to Mn²⁺. What volume of 0.020 M KMnO₄ is needed for 50 mL of 0.10 M FeC₂O₄?
1. A150 mL
2. B50 mL
3. C375 mL
4. D100 mL
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Question 14 · Medium
Which of the following is NOT a redox reaction?
1. A2Na + 2H₂O → 2NaOH + H₂
2. BCl₂ + 2KI → 2KCl + I₂
3. CCaCO₃ → CaO + CO₂
4. D2KClO₃ → 2KCl + 3O₂
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Question 15 · Medium
When white phosphorus is warmed with aqueous alkali, it reacts as follows: P₄ + 3OH⁻ + 3H₂O → PH₃ + 3H₂PO₂⁻ The oxidation number of phosphorus changes from 0 to
1. A−3 and +3
2. B−3 and +1
3. C+3 and +1
4. D−3 and +5
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Question 16 · Medium
Arrange the metals below in increasing order of their tendency to lose electrons, using these observations: zinc displaces copper from CuSO₄ solution, copper displaces silver from AgNO₃ solution, and magnesium displaces zinc from ZnSO₄ solution. A. Ag B. Zn C. Cu D. Mg Choose the correct answer from the options given below.
1. AD, B, C, A
2. BC, A, B, D
3. CA, C, B, D
4. DA, B, C, D
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Question 17 · Medium
Chlorine reacts with hot, concentrated NaOH: 3Cl₂ + 6OH⁻ → 5Cl⁻ + ClO₃⁻ + 3H₂O. What is the ratio of chlorine atoms reduced to chlorine atoms oxidised?
1. A1 : 1
2. B1 : 5
3. C3 : 1
4. D5 : 1
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Question 18 · Hard
Consider the following statements. A. In NH₄NO₂ → N₂ + 2H₂O, nitrogen is both oxidised and reduced. B. Every decomposition reaction is a redox reaction. C. The reaction 2Cu⁺(aq) → Cu²⁺(aq) + Cu(s) is a disproportionation. D. The oxidation number of oxygen in KO₂ is −1/2. Choose the correct answer from the options given below.
1. AA, B, C and D
2. BA and C only
3. CA, C and D only
4. DC and D only
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Question 19 · Easy
In the reaction H₂S + Cl₂ → 2HCl + S, which species is oxidised?
1. AS
2. BHCl
3. CH₂S
4. DCl₂
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Question 20 · Hard
When the equation Cr₂O₇²⁻ + H₂S + H⁺ → Cr³⁺ + S + H₂O is balanced with the smallest whole-number coefficients, how many H⁺ ions are needed per Cr₂O₇²⁻ ion?
1. A14
2. B6
3. C7
4. D8
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Question 21 · Easy
What is the oxidation number of manganese in potassium manganate, K₂MnO₄?
1. A+6
2. B+7
3. C+4
4. D+5
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Question 22 · Medium
In acidic solution, 2MnO₄⁻ + 5H₂O₂ + 6H⁺ → 2Mn²⁺ + 5O₂ + 8H₂O. What volume of 0.020 M KMnO₄ is needed to react completely with 25 mL of 0.050 M H₂O₂?
1. A62.5 mL
2. B156 mL
3. C10 mL
4. D25 mL
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Question 23 · Medium
Given below are two statements: Statement I: In a disproportionation reaction, an element in one oxidation state is simultaneously oxidised and reduced. Statement II: Fluorine undergoes disproportionation when it reacts with alkali. Choose the correct answer.
1. AStatement I is incorrect but Statement II is correct
2. BStatement I is correct but Statement II is incorrect
3. CBoth Statement I and Statement II are incorrect
4. DBoth Statement I and Statement II are correct
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Redox Reactions

Given below are two statements: Statement I: The oxidation number of oxygen in OF₂ is −2. Statement II: The oxidation number of hydrogen in NaH is +1. Choose the correct answer.

Exactly 1.26 g of H₂C₂O₄·2H₂O is dissolved to make 250 mL of solution. A 25 mL portion is acidified and titrated with 0.02 M KMnO₄: 2MnO₄⁻ + 5C₂O₄²⁻ + 16H⁺ → 2Mn²⁺ + 10CO₂ + 8H₂O What volume of KMnO₄ is needed? (H₂C₂O₄·2H₂O = 126, H₂C₂O₄ = 90)

In the reaction Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s), which species acts as the oxidising agent?

An acidified permanganate solution oxidises iron(II): a MnO₄⁻ + b Fe²⁺ + c H⁺ → Mn²⁺ + Fe³⁺ + H₂O (not balanced) For the balanced ionic equation in lowest whole numbers, what are a, b and c?

What is the oxidation number of phosphorus in H₃PO₂?

Disproportionation occurs in which of the following reactions?

Arrange the following in increasing order of the oxidation number of nitrogen. A. NO₂ B. HNO₃ C. N₂O D. NH₃

In oxygen difluoride, OF₂, the oxidation number of oxygen is

In a neutral or weakly basic solution, permanganate oxidises sulphite to sulphate and is itself reduced to MnO₂. When the ionic equation a MnO₄⁻ + b SO₃²⁻ + c H₂O → d MnO₂ + e SO₄²⁻ + f OH⁻ is balanced with the smallest whole numbers, what are a, b and f?

Acidified potassium dichromate oxidises Fe²⁺ as follows: Cr₂O₇²⁻ + 6Fe²⁺ + 14H⁺ → 2Cr³⁺ + 6Fe³⁺ + 7H₂O What volume of 0.10 M K₂Cr₂O₇ is needed to oxidise all the Fe²⁺ in 30 mL of 0.20 M FeSO₄ solution?

Match List-I with List-II. List-I (element in compound) A. Mn in KMnO₄ B. Cr in K₂Cr₂O₇ C. S in Na₂S₂O₃ D. Fe in Fe₃O₄ List-II (oxidation number, average where needed) I. +6 II. +8/3 III. +2 IV. +7 Choose the correct answer.

In the reaction 2S₂O₃²⁻ + I₂ → S₄O₆²⁻ + 2I⁻, the average oxidation number of sulphur changes from

Acidified KMnO₄ oxidises iron(II) oxalate, FeC₂O₄, completely: Fe²⁺ goes to Fe³⁺ and C₂O₄²⁻ goes to CO₂, while MnO₄⁻ is reduced to Mn²⁺. What volume of 0.020 M KMnO₄ is needed for 50 mL of 0.10 M FeC₂O₄?

Which of the following is NOT a redox reaction?

When white phosphorus is warmed with aqueous alkali, it reacts as follows: P₄ + 3OH⁻ + 3H₂O → PH₃ + 3H₂PO₂⁻ The oxidation number of phosphorus changes from 0 to

Chlorine reacts with hot, concentrated NaOH: 3Cl₂ + 6OH⁻ → 5Cl⁻ + ClO₃⁻ + 3H₂O. What is the ratio of chlorine atoms reduced to chlorine atoms oxidised?

In the reaction H₂S + Cl₂ → 2HCl + S, which species is oxidised?

When the equation Cr₂O₇²⁻ + H₂S + H⁺ → Cr³⁺ + S + H₂O is balanced with the smallest whole-number coefficients, how many H⁺ ions are needed per Cr₂O₇²⁻ ion?

What is the oxidation number of manganese in potassium manganate, K₂MnO₄?

In acidic solution, 2MnO₄⁻ + 5H₂O₂ + 6H⁺ → 2Mn²⁺ + 5O₂ + 8H₂O. What volume of 0.020 M KMnO₄ is needed to react completely with 25 mL of 0.050 M H₂O₂?

Given below are two statements: Statement I: In a disproportionation reaction, an element in one oxidation state is simultaneously oxidised and reduced. Statement II: Fluorine undergoes disproportionation when it reacts with alkali. Choose the correct answer.