While preparing crystals of Mohr's salt, dil H₂ SO₄ is added to a mixture of ferrous sulphate and ammonium sulphate, before dissolving this mixture in water, dil H₂SO₄ is added here to :

Solution

Chemistry ma’am · 3 steps

Ferrous sulphate is the one that hydrolyses, so a little acid keeps Fe²⁺ safe in solution.

1. What goes wrong. FeSO₄ is the salt of a weak base, Fe(OH)₂, and a strong acid. In water, Fe²⁺ hydrolyses and forms basic iron salts, so the solution turns turbid.

   Fe²⁺ + 2H₂O ⇌ Fe(OH)₂ + 2H⁺

2. Role of the acid. Adding dilute H₂SO₄ raises [H⁺]. By Le Chatelier's principle this pushes the equilibrium to the left, so hydrolysis is suppressed and clear, pale green crystals can form.
3. So. The dilute acid is added to prevent the hydrolysis of ferrous sulphate, option (a).

The idea to keep. Salts of weakly basic metal ions such as Fe²⁺, Fe³⁺ and Al³⁺ hydrolyse in water. A little of the matching acid pushes that equilibrium back and keeps the solution clear.

Answer from the NTA final answer key. Solution by PrepWiser.