NEET (UG) 2024 — Question 85

Mohr's salt is $\ce{(NH4)2Fe(SO4)2.6H2O}$. In aqueous solution, the $\ce{Fe^{2+}}$ ion undergoes hydrolysis: $\ce{Fe^{2+} + 2H2O <=> Fe(OH)2 + 2H+}$. Adding dilute $\ce{H2SO4}$ increases the $\ce{H+}$ concentration, shifting the equilibrium to the left (common ion effect) and suppressing hydrolysis. Dilute $\ce{HCl}$ would introduce $\ce{Cl-}$ ions that can form complexes with $\ce{Fe^{2+}}$, concentrated $\ce{H2SO4}$ is too strong and may oxidise $\ce{Fe^{2+}}$ to $\ce{Fe^{3+}}$, and dilute $\ce{HNO3}$ is an oxidising agent that would also oxidise $\ce{Fe^{2+}}$. Hence, dilute sulphuric acid is the correct choice.