Read the passage carefully and answer the Questions.
\( KMnO_4 \) is prepared by the fusion of \( MnO_2 \) with an alkali metal hydroxide and an oxidizing agent like \( KNO_3 \) to give a dark- green manganate ion which disproportionates to give permanganate as follows.
\( 2MnO_2+ 4KOH + O_2→2K_2MnO_4+2H_2O \)
\( 3K_2MnO_4+ 4H^+→2KMnO_4 + MnO_2 + 2H_2O \)
On heating \( KMnO_4 \) decomposes at 513 K to give \( K_2MnO_4 \). Permanganate ion is tetrahedral and diamagnetic. Acidified \( KMnO_4 \) acts as a strong oxidizing agent which oxidizes oxalic acid, ferrous ions, nitrite ion and iodides.
\( HCl \) is not used to make the medium acidic in oxidation reaction of \( KMnO_4 \) in acidic medium, because....
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → \(KMnO_4\) oxidizes \(HCl\) into \(Cl_2\) which is also an oxidizing agent
The acid used to make a permanganate solution acidic must not react with permanganate itself. Acidified permanganate (\( E^\circ = +1.52 \) V for \( \ce{MnO4^-}/\ce{Mn^2+} \)) is a stronger oxidising agent than chlorine (\( E^\circ = +1.36 \) V for \( \ce{Cl2}/\ce{Cl^-} \)), so it oxidises the chloride ions of HCl to chlorine:
\( \displaystyle \ce{2MnO4^- + 16H^+ + 10Cl^- -> 2Mn^2+ + 5Cl2 + 8H2O} \)
Part of the permanganate is used up in this side reaction, and the chlorine formed is itself an oxidising agent that can oxidise the substance being titrated. The volume of permanganate used then does not match the reducing agent, so the result is wrong. For this reason dilute sulphuric acid, which permanganate cannot oxidise, is used instead.
Why the other options are incorrect
Option (1) \(KMnO_4\) act as reducing agent in the presence of \(HCl\) — Incorrect. Manganese in \( \ce{KMnO4} \) is in its highest oxidation state, +7, so it can only be reduced; it oxidises HCl and is never a reducing agent.
Option (3) \(KMnO_4\) is a weaker oxidizing agent than \(HCl\) — Incorrect. It is the other way round: permanganate is strong enough to oxidise HCl, and that is the very reason HCl is avoided.
Option (4) both \(HCl\) and \(KMnO_4\) act as oxidizing agent — Incorrect. In this reaction HCl is the reducing agent (\( \ce{Cl^-} \) loses electrons to give \( \ce{Cl2} \)); only permanganate is the oxidising agent.
Note 1: It seems that there is a mistake in the original NTA paper. The second equation of the passage is printed as 3K2MnO4 + 4H+ → 2KMnO4 + MnO2 + 2H2O, in which the charges do not balance; it should be 3MnO42− + 4H+ → 2MnO4− + MnO2 + 2H2O.
NCERT (2026-27 edition):
Oxides and Oxoanions of Metals – …
Note 2: The same/similar question has been asked in different years by NTA as per the details given below:
| 2026 | 2025 | 2024 | 2023 | 2022 |
|---|---|---|---|---|
| – | 03-06 S1 13-05 S1 | – | – | – |