UPSC Prelims 2002 Q110: Science & Technology – Chemistry
Match List I (Oxidation number) with List II (The element) and select the correct answer
using the codes given below the lists:
List I (Oxidation number) - List II (The element)
A. 2 - 1. Oxidation number of Mn in MnO₂
B. 3 - 2. Oxidation number of S in H₂S₂O₇
C. 4 - 3. Oxidation number of Ca in CaO₂
D. 6 - 4. Oxidation number of Al in NaAlH₄
- (a) A - 3, B - 4, C - 1, D - 2
- (b) A - 4, B - 3, C - 1, D - 2
- (c) A - 3, B - 4, C - 2, D - 1
- (d) A - 4, B - 3, C - 2, D - 1
Answer: (a) A - 3, B - 4, C - 1, D - 2
Explanation
Matching oxidation numbers with the compounds
| Compound | Oxidation number |
|---|---|
| Ca in CaO₂ (peroxide) | +2, so A (2) matches 3 |
| Al in NaAlH₄ | +3 (Na +1, H −1), so B (3) matches 4 |
| Mn in MnO₂ | +4, so C (4) matches 1 |
| S in H₂S₂O₇ | +6, so D (6) matches 2 |
In calcium peroxide Ca is +2 and the peroxide group is −2. In NaAlH₄, Na is +1 and each H is −1, so Al is +3. In MnO₂, Mn is +4. In H₂S₂O₇ (oleum), S is +6. The code is A-3, B-4, C-1, D-2.
Answer: (a) A - 3, B - 4, C - 1, D - 2.
Why each option is right or wrong
- (a) CORRECT. A-3, B-4, C-1, D-2 matches the oxidation numbers.
- (b) Incorrect. It gives Al in NaAlH₄ a value of +2 instead of +3.
- (c) Incorrect. It swaps the values for Mn in MnO₂ and S in H₂S₂O₇.
- (d) Incorrect. It swaps the values for Mn in MnO₂ and S in H₂S₂O₇ and for Ca and Al.
Key point
Oxidation number rules: H is −1 in metal hydrides, O is −1 in peroxides, and the sum equals the charge.
Remember
CaO₂ gives +2; NaAlH₄ gives +3; MnO₂ gives +4; H₂S₂O₇ gives +6.
Common mistakes
- Treating oxygen as −2 in calcium peroxide, giving Ca +4.
- Taking hydrogen as +1 in NaAlH₄, although it is −1 in a metal hydride.
- Forgetting the two sulphur atoms in H₂S₂O₇.
Exam tip
Work out MnO₂ (+4) and H₂S₂O₇ (+6) first, then match the rest.
Sources
- NCERT Class XI Chemistry — Redox Reactions
- J. D. Lee — Concise Inorganic Chemistry (Oxidation states)