Question

While preparing crystals of Mohr's salt, dil. $H_2SO_4$ is added to a mixture of ferrous sulphate and ammonium sulphate, before dissolving this mixture in water, dil. $H_2SO_4$ is added here to:

• A. prevent the hydrolysis of ferrous sulphate
• B. prevent the hydrolysis of ammonium sulphate
• C. make the medium strongly acidic
• D. increase the rate of formation of crystals

Answer 1

Answer: (A)

prevent the hydrolysis of ferrous sulphate

Answer 2

When preparing Mohr’s salt, $\text{H}_2\text{SO}_4$ is added for the following reasons:
Step 1: Understanding the role of $\text{H}_2\text{SO}_4$
Ferrous sulphate is prone to hydrolysis in aqueous solutions, especially when exposed to atmospheric oxygen. The hydrolysis leads to the formation of ferric hydroxide, which can contaminate the Mohr’s salt crystals.
Adding dilute sulphuric acid prevents the hydrolysis of ferrous sulphate by maintaining an acidic medium, which stabilizes the ferrous ions and prevents oxidation or decomposition.
Step 2: Reason for not choosing other options
Option (2): Ammonium sulphate does not hydrolyze under normal conditions, so preventing its hydrolysis is irrelevant.
Option (3): Although adding $\text{H}_2\text{SO}_4$ makes the medium acidic, the main purpose is to prevent hydrolysis.
Option (4): The rate of formation of crystals is not directly influenced by $\text{H}_2\text{SO}_4$ but rather by cooling and saturation.
Conclusion:
The correct reason for adding $\text{H}_2\text{SO}_4$ is to prevent the hydrolysis of ferrous sulphate.