Identify the compound being prepared

Mohr's salt is: $\mathrm{FeSO_4\cdot (NH_4)_2SO_4\cdot 6H_2O}$

It is prepared from ferrous sulphate and ammonium sulphate in acidic medium.

Why is dilute $\mathrm{H_2SO_4}$ added?

Ferrous sulphate contains the salt of a strong acid and a weak base: $\mathrm{Fe(OH)_2} \text{ is a weak base}$ Hence, $\mathrm{FeSO_4}$ undergoes hydrolysis in water, making the solution unstable.

The hydrolysis may be represented as: $\mathrm{Fe^{2+} + 2H_2O \rightleftharpoons Fe(OH)_2 + 2H^+}$

On adding dilute $\mathrm{H_2SO_4}$, the medium becomes acidic, and by Le Chatelier's principle, hydrolysis of $\mathrm{Fe^{2+}}$ is suppressed.

Check each option

• A: prevent the hydrolysis of ferrous sulphate
  Correct. This is the main purpose.

• B: prevent the hydrolysis of ammonium sulphate
  Incorrect. Ammonium sulphate does not present the relevant problem here.

• C: increase the rate of formation of crystals
  Incorrect. Acid is not added for speeding crystallization.

• D: make the medium strongly acidic
  Incorrect. The aim is not to make it strongly acidic, but sufficiently acidic to suppress hydrolysis.

Final answer

The dilute $\mathrm{H_2SO_4}$ is added to prevent the hydrolysis of ferrous sulphate.

Therefore, the correct option is: $\boxed{\text{A}}$