Question

Which amongst the following gases can decolourise pink coloured acidified $KMn{O}_4$?

• A. Sulphur dioxide
• B. Carbon dioxide
• C. Nitrogen dioxide
• D. Hydrogen sulphide

Answer 1

The correct option is A Sulphur dioxide

$S{O}_2$, when passed through pink-coloured acidified $KMn{O}_4$, changes it to colourless. The reaction occurs as given below:

$2KMn{O}_4\left(aq\right)+2H_{2}O\left(l\right)+5S{O}_2\left(g\right)\to K_{2}S{O}_4\left(aq\right)+2MnS{O}_4\left(aq\right)+2H_{2}S{O}_4\left(aq\right)$

The formation of $MnS{O}_4$ (Manganese sulphate) turns the solution colourless.

This is a confirmatory test for sulphur dioxide gas as the other gases like $C{O}_2$, $CO$ and $H_{2}S$ has no decolourising effect on $KMn{O}_4$.