During the preparation of Mohr's salt solution (Ferrous ammonium sulphate), which of the following acid is added to prevent hydrolysis of Fe2+ ion?
- dilute hydrochloric acid
- concentrated sulphuric acid
- dilute nitric acid
- dilute sulphuric acid
Answer
(4) dilute sulphuric acid
Solution
Fe2+ ions hydrolyse in water: Fe2+ + 2H2O Fe(OH)2 + 2H+
Adding H+ shifts this equilibrium to the left, so a little dilute H2SO4 is added (it supplies the common SO42- ion of the salt).
HCl would introduce Cl- impurity, HNO3 would oxidise Fe2+ to Fe3+, and concentrated H2SO4 is also an oxidising agent.
Answer: (4)