JEE Main 2022 · 28 July · evening · Single correct · +4 / −1
Given below are two statements : One is labelled as Assertion A and the other is labelled as Reason R
Assertion A : Permanganate titrations are not performed in presence of hydrochloric acid.
Reason R : Chlorine is formed as a consequence of oxidation of hydrochloric acid.
In the light of the above statements, choose the correct answer from the options given below
- (a) Both A and R are true and R is the correct explanation of A
- (b) Both A and R are true but R is NOT the correct explanation of A
- (c) A is true but R is false
- (d) A is false but R is true
Answer and solution
Answer (a) Both A and R are true and R is the correct explanation of A
Solution
Chemistry ma’am · 4 steps
In HCl, permanganate wastes itself oxidising chloride to chlorine, so the titre reading comes out wrong.
- Why acid is needed. KMnO₄ is a strong oxidising agent in acid, where MnO₄⁻ is reduced to Mn²⁺. So these titrations must be done in an acidic medium.
- What HCl does. MnO₄⁻ is strong enough to oxidise chloride ions. The Cl⁻ in hydrochloric acid gives off chlorine gas, so Reason R is true.
2MnO₄⁻ + 16H⁺ + 10Cl⁻ → 2Mn²⁺ + 5Cl₂ + 8H₂O
- Effect on titration. Part of the permanganate is used up on the acid instead of on the substance being titrated. The volume of KMnO₄ used comes out too high and the result is wrong. That is why HCl is avoided and dilute H₂SO₄ is used, which permanganate cannot oxidise.
- So. Both statements are true, and the formation of chlorine is exactly why HCl is avoided. The answer is (a).
The idea to keep. For a titration acid, choose one that the oxidant cannot attack. KMnO₄ oxidises Cl⁻ to Cl₂, so use dilute H₂SO₄, never HCl.
Answer from the NTA final answer key. Solution by PrepWiser.