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

To prepare Mohr's salt solution, preventing Fe²⁺ ion hydrolysis is crucial. Hydrolysis occurs when water destabilizes a compound. Ferrous ions (Fe²⁺) easily oxidize and hydrolyze, particularly in alkaline conditions. An acidic environment is maintained to prevent this, with dilute sulphuric acid being the optimal choice.

Dilute sulphuric acid is preferred for the following reasons:

• It establishes the necessary acidic medium for Fe²⁺ ion stability.
• It avoids introducing ions that could precipitate with Fe²⁺, thus ensuring a clear solution.
• As a strong acid, sulphuric acid dissociates completely, creating an effective acidic environment without volatility or decomposition risks.
• Alternative acids like hydrochloric and nitric acid can produce volatile by-products or oxidizing agents, leading to undesired reactions or the oxidation of Fe²⁺ to Fe³⁺.

Consequently, dilute sulphuric acid is the appropriate selection among available options to inhibit Fe²⁺ ion hydrolysis during Mohr's salt solution preparation.