Question

Name the gas that readily decolourises the acidified $KMn{O_4}$ solution.
A. $C{O_2}$
B. $S{O_2}$
C. $N{O_2}$
D. ${P_2}{O_5}$

Answer 1

One needs to understand the qualitative analysis of potassium permanganate to identify the gas that would decolourise the acidified potassium permanganate solution.

Complete step by step solution:
Permanganate and manganate solutions can be prepared by methods such as manganate can be obtained by boiling potassium permanganate with conc.KOH.
$4KMn{O_4} + 4KOH\xrightarrow{{boiling}}4{K_2}Mn{O_4} + 2{H_2}O + 2{O_2}$
The above green solution is quite stable in alkali, but decomposes in pure water and in the presence of acids, depositing manganate oxide and giving a purple solution of permanganate.
$3{K_2}Mn{O_4} + 2{H_2}O \rightleftharpoons 2KMn{O_4} + Mn{O_2} \downarrow + 4KOH$
This reaction is also a method of preparation of $KMn{O_4}$ which is purple in colour.
For use as an oxidizing agent, $KMn{O_4}$ should be stored in dark bottles and standardized just before use due to the following reasons:
1. It is not a primary standard since it is difficult to get it in a high degree of purity and free form traces of $Mn{O_2}$.
2. It is slowly reduced to $Mn{O_2}$ especially in the presence of light or acid.
All available manganate and permanganate salts are soluble in water. There are several tests of $MnO_4^ -$ which are not shown by $MnO_4^{2 - }$. Because $MnO_4^{2 - }$ is stable in solution with pH=12 only. For example, the purple colour of $MnO_4^ -$ is discharged by several reducing agents as is evident from the reaction given below:
$MnO_4^ - + S{O_2}\xrightarrow{{{H^ + }}}M{n^{2 + }} + SO_4^{2 - } + {H_2}O$
In this test permanganate radical gives a green solution on treatment with hot and concentrated NaOH. The purple colour of permanganate solution is restored by acidification or by dilution with large amounts of water.

**Hence we can conclude that the correct option is B.

Note: **
These solutions need to be handled with care and should be performed under expert supervision. Since the heating effect of potassium permanganate gives black residue.