JEE Main 2020 · 7 January · morning · Single correct · +4 / −1

Oxidation number of potassium in K₂O. K₂O₂ and KO₂ respectively is :

  1. (a) +1, +2 and + 4
  2. (b) +1, +1 and + 1
  3. (c) +2, +1 and + 1/2
  4. (d) +1, +4, and +2
Answer and solution

Answer (b) +1, +1 and + 1

Solution

Chemistry ma’am · 3 steps

Fix potassium first: a Group 1 metal is always +1, so let oxygen take whatever value is left.

  1. Start with potassium. Potassium is an alkali metal with one valence electron, and it always loses that electron in its compounds. So K is +1 in every one of these. Oxygen is the element whose oxidation number changes here.
    CompoundOxygen speciesOxidation number of OOxidation number of K
    K₂O (oxide)O²⁻-22x - 2 = 0, so x = +1
    K₂O₂ (peroxide)O₂²⁻-12x - 2 = 0, so x = +1
    KO₂ (superoxide)O₂⁻-1/2x - 1 = 0, so x = +1
  2. Why oxygen changes. In a peroxide the two O atoms share a charge of -2, so each is -1. In a superoxide the two O atoms share only -1, so each is -1/2. The total charge on the oxygen part always matches the number of K atoms.
  3. So. Potassium is +1, +1 and +1 in the three compounds, which is option (b).

The idea to keep. Alkali metals are always +1 and alkaline earth metals always +2 in their compounds. In oxides, peroxides and superoxides, oxygen is the one that changes: -2, -1 and -1/2.

Answer from the NTA final answer key. Solution by PrepWiser.

Study Redox Reactions

Asked about 90 times in JEE Main since 2020. This question is from 2020.