Question

Mohr’s salt is dissolved in dil ${{H}_{2}}S{{O}_{4}}$instead of distilled water to?
A. Enhance the rate of dissolution
B. Prevent cationic hydrolysis
C. Increase the rate of ionisation
D. Increase its reducing strength

Answer 1

Hint: Mohr’s salt is a double salt. It has the tendency to dissociate into its constituent ions if present in aqueous solution. But that cannot be allowed to happen when the standard solution is being prepared. Moreover, oxidation is also not preferred.

Complete answer:
Let us get to know what Mohr’s salt is. Molr’s salt is used in the branch of analytical chemistry Mohr’s salt can be said as a double sulphate salt (tutton’s salts) which is made by dissolving ammonium sulphate and Iron sulphate in proportions with respect to molar concentration with some small amount of ${{H}_{2}}S{{O}_{4}}$. They are mixed together and crystalized by means of evaporation. It is used for purposes like iron plating, and also as stable source of ferrous ions as it is more stable due to the slight acidity that the ammonium sulphate imparts to the solution and helps to prevent oxidation to $F{{e}^{3}}^{+}$. But as Mohr’s salt is a double salt, it would have dissociated in all it’s constituent ions if we placed it in distilled water. So, conc ${{H}_{2}}S{{O}_{4}}$ is added so that the cationic hydrolysis is paused or stopped. It is necessary to stop the hydrolysis of the salt, or else it will be difficult for the preparation of the standard solution. So, we obtain option B as our answer.

Note:
Apart from the cationic hydrolysis, the application of ${{H}_{2}}S{{O}_{4}}$ has one more use. The concentrated ${{H}_{2}}S{{O}_{4}}$ prevents the oxidation of $F{{e}^{2+}}$ to $F{{e}^{3+}}$ ions. Oxidation can however take place at high pH values. ${{H}_{2}}S{{O}_{4}}$acts both as an acid as well as an oxidising agent.