Question

Which of the following acids cannot be used to provide ${{H}^{+}}$ ion for the estimation of Mohr’s salt by $KMn{{O}_{4}}$?
A) $HCl$
B) $HN{{O}_{3}}$
C) ${{H}_{2}}S{{O}_{4}}$
D) $HI$

Answer 1

This is a redox titration reaction. There are three main components of this reaction, Mohr’s salt, $KMn{{O}_{4}}$and an acidic medium. While choosing the correct acidic medium for titration, keep in mind that medium should not be a strong reducing or a strong oxidizing agent as it will take part in reaction and will give unsatisfactory results.

Complete step-by-step answer: A group of acids that may or may not provide ${{H}^{+}}$ ion for estimation of Mohr’s salt by $KMn{{O}_{4}}$ first important step to solve this question is to first understand what is titration method given in question. $KMn{{O}_{4}}$is a strong oxidant in presence of ${{H}_{2}}S{{O}_{4}}$. Mohr’s salt is a double salt that forms a single crystalline structure, also known as ferrous ammonium sulphate.
In this titration, Mohr’s salt acts as a reducing agent and $KMn{{O}_{4}}$acts as an oxidizing agent. So the reaction between Mohr’s salt as well as potassium permanganate is known as redox reaction. In this redox reaction, ferrous ion from Mohr’s salt gets oxidized and pink colored manganese present in potassium permanganate which is in $+7$ oxidation state gets reduced to colorless Mn2+ state.
$HCl$is a mild reducing agent and it reacts with some of indicator that is $KMn{{O}_{4}}$ solution so the final value of the indicator used come to be more than the actual value and disturbs the experiment. Hence, $HCl$ cannot be used to provide ${{H}^{+}}$ ion for estimation of Mohr’s salt by $KMn{{O}_{4}}$
Similarly, $HN{{O}_{3}}$is also a powerful oxidizing agent along with $KMn{{O}_{4}}$so it also plays its role in oxidation which causes a decline in value of $KMn{{O}_{4}}$ used. Hence, $HN{{O}_{3}}$ is also useless in this process. $HI$is a strong reducing agent owing to the oxidizing nature of halogens. It can cause additional reduction which will not provide satisfactory results. As ${{H}_{2}}S{{O}_{4}}$ dilute is ideal for redox titration because it is neither an oxidizing agent nor a reducing agent.

Therefore correct answer are A, B and D i.e. $HCl$, $HN{{O}_{3}}$ and $HI$ respectively.

Note: This titration is based upon oxidation-reduction titrations. When ferrous ammonium sulphate solution is titrated against potassium permanganate in presence of acidic medium by sulfuric acid. Acidic mediums are necessary for prevention of precipitation of manganese oxide. Here $KMn{{O}_{4}}$ acts as a self-indicator and this titration is called permanganate titration.