Question

During the preparation of Mohr’s salt solution (Ferrous ammonium sulphate), which of the following acid is added to prevent hydrolysis of Fe²⁺ ion?

• A. dilute hydrochloric acid
• B. concentrated sulphuric acid
• C. dilute nitric acid
• D. dilute sulphuric acid

Answer 1

The Correct Option is D

In the preparation of Mohr’s salt solution, it is essential to prevent the hydrolysis of the Fe²⁺ ions. Hydrolysis is a reaction where water reacts with a compound, affecting its stability. Ferrous ions (Fe²⁺) are prone to oxidation and hydrolysis, especially in an alkaline medium. To prevent these reactions, an acidic environment is maintained. The most suitable acid for this purpose is dilute sulphuric acid.

Here's why we use dilute sulphuric acid:

• Dilute sulphuric acid maintains an acidic medium, which is necessary for the stability of Fe²⁺ ions.
• It does not introduce any additional ions that could precipitate with Fe²⁺, ensuring a clear solution.
• Sulphuric acid is a strong acid, dissociating completely in water, providing an effective acidic environment without the risk of volatility or decomposition.
• Other acids, such as hydrochloric and nitric acid, could introduce volatile by-products or oxidizing agents, which can lead to unwanted side reactions or oxidation of the ferrous ions to ferric ions (Fe³⁺).

Therefore, among the given options, dilute sulphuric acid is the correct choice to prevent the hydrolysis of Fe²⁺ ion during the preparation of Mohr’s salt solution.