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 :

Mohr’s salt is ferrous ammonium sulphate: $$FeSO_4 \cdot (NH_4)_2SO_4 \cdot 6H_2O$$. When preparing its crystals, a mixture of ferrous sulphate ($$FeSO_4$$) and ammonium sulphate ($$(NH_4)_2SO_4$$) is dissolved in water, and dilute $$H_2SO_4$$ is added.

In aqueous solution, ferrous sulphate undergoes hydrolysis, forming a brown/green basic ferrous sulphate precipitate via the reaction $$ Fe^{2+} + 2H_2O \rightleftharpoons Fe(OH)_2 + 2H^+. $$ Adding dilute $$H_2SO_4$$ increases the $$H^+$$ concentration, shifting the equilibrium to the left (Le Chatelier’s principle) and keeping the ferrous ions in solution.

Furthermore, the acidic medium prevents oxidation of $$Fe^{2+}$$ to $$Fe^{3+}$$. Thus, dilute $$H_2SO_4$$ is added to prevent the hydrolysis of ferrous sulphate, corresponding to Option (1).