Question:hard

Name the gas that can readily decolourise acidified $KMnO_4$ solution :

Updated On: May 7, 2026
  • $SO_2$
  • $NO_2$
  • $P_2O_5$
  • $CO_2$
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The Correct Option is A

Solution and Explanation

In this question, we need to identify the gas that can readily decolourise acidified potassium permanganate (\(KMnO_4\)) solution. Let's analyze each option to determine the correct answer:

  1. $SO_2$ (Sulfur Dioxide): \(SO_2\) is a reducing agent and can decolourise acidified \(KMnO_4\) by converting \(MnO_4^-\) ions to colorless \(Mn^{2+}\). The reaction is as follows:
    \[ 2 KMnO_4 + 5 SO_2 + 2 H_2O \rightarrow 2 MnSO_4 + K_2SO_4 + 2 H_2SO_4 \]
    Since \(SO_2\) reduces \(KMnO_4\), it can readily decolourise the purple solution of \(KMnO_4\).
  2. $NO_2$ (Nitrogen Dioxide): \(NO_2\) is not known to decolourise acidified \(KMnO_4\) as effectively as \(SO_2\). It is less commonly involved in these reactions.
  3. $P_2O_5$ (Phosphorus Pentoxide): \(P_2O_5\) is not a gas; it is a solid at room temperature. It is irrelevant in the context of reducing agents for \(KMnO_4\).
  4. $CO_2$ (Carbon Dioxide): \(CO_2\) is not a reducing agent. It does not react significantly with \(KMnO_4\) to cause decolourisation.

Thus, the gas that can readily decolourise acidified \(KMnO_4\) solution is $SO_2$. This is because \(SO_2\) is a strong reducing agent and reacts effectively with \(KMnO_4\) to change its oxidation state, leading to decolourisation.

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