Q132 / 200 · MHT CET - Medical 2026 (24 April Shift 1) · Chemistry
Identify the gas that readily decolourises acidified $\text{KMnO}_4$ solution.
- A$\text{CO}_2$
- Correct answerB$\text{SO}_2$
- C$\text{NO}_2$
- D$\text{H}_2\text{S}$
Solution
Acidified $\text{KMnO}_4$ acts as a strong oxidizing agent. $\text{SO}_2$ is a good reducing agent and readily reduces the purple-coloured acidified $\text{KMnO}_4$ solution to colourless $\text{Mn}^{2+}$ ions.
The chemical reaction is given by:
$2\text{KMnO}_4 + 5\text{SO}_2 + 2\text{H}_2\text{O} \rightarrow \text{K}_2\text{SO}_4 + 2\text{MnSO}_4 + 2\text{H}_2\text{SO}_4$
$\text{CO}_2$ does not react with $\text{KMnO}_4$. While $\text{H}_2\text{S}$ can also reduce $\text{KMnO}_4$, it produces a milky suspension of colloidal sulphur, whereas $\text{SO}_2$ yields a clear, colourless solution. Thus, $\text{SO}_2$ is the gas that readily decolourises the solution.
Answer: $\text{SO}_2$
The chemical reaction is given by:
$2\text{KMnO}_4 + 5\text{SO}_2 + 2\text{H}_2\text{O} \rightarrow \text{K}_2\text{SO}_4 + 2\text{MnSO}_4 + 2\text{H}_2\text{SO}_4$
$\text{CO}_2$ does not react with $\text{KMnO}_4$. While $\text{H}_2\text{S}$ can also reduce $\text{KMnO}_4$, it produces a milky suspension of colloidal sulphur, whereas $\text{SO}_2$ yields a clear, colourless solution. Thus, $\text{SO}_2$ is the gas that readily decolourises the solution.
Answer: $\text{SO}_2$
Hence, the correct answer is option: 'B'.