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During the preparation of Mohr's salt solution (Ferrous ammonium sulphate), which of the following acid is added to prevent hydrolysis of Fe²⁺ ion?
Mohr’s salt is a SULPHATE double salt! Dilute H2SO4 prevents hydrolysis of Fe²⁺ to Fe(OH)2 without introducing foreign anions or oxidizing Fe²⁺!
When Mohr’s salt dissolves in neutral water, the Fe²⁺ cation acts as a weak conjugate acid and undergoes hydrolysis, producing basic salts / insoluble Fe(OH)2 which causes the solution to turn brownish and turbid:
By adding dilute sulphuric acid, the high concentration of hydronium ions (H⁺) shifts the hydrolysis equilibrium to the left according to Le Chatelier’s principle, completely preventing the precipitation of Fe(OH)2 and maintaining a clear, transparent solution of Fe²⁺:
Evaluating the alternative acids: - Dilute HNO3: Nitric acid is a strong oxidizing agent that oxidizes ferrous (Fe²⁺) to ferric (Fe³⁺), destroying the standard reducing solution: 3 Fe²⁺ + NO3⁻ + 4 H⁺ ⟶ 3 Fe³⁺ + NO + 2 H2O. - Dilute HCl: In redox titrations with KMnO4, hydrochloric acid is oxidized by permanganate to chlorine gas (2 Cl⁻ ⟶ Cl2 + 2e⁻), consuming titrant and producing false titer values. - Concentrated H2SO4: Charring agent; unnecessary and dangerous for aqueous volumetric solutions. Hence, dilute H2SO4 is the unique, ideal choice.
Diagnose every choice: Know exactly why the right option works and why distractors fail.
Oxidizes Fe²⁺ to Fe³⁺, destroying the reducing capacity of Mohr’s salt.
Supplies H⁺ to suppress hydrolysis, shares common sulphate ion, and does not interfere with KMnO4 titrations.
Cl⁻ is oxidized to Cl2 by KMnO4 during redox titration.
Excessively hazardous and unnecessary; only dilute acid is specified.
Students often think any strong mineral acid will work. In redox titrations, KMnO4 oxidizes HCl to Cl2! Only dilute H2SO4 provides the non-interfering acidic environment.
"Dilute H2SO4 is added during the preparation of Mohr’s salt solution to prevent cationic hydrolysis of Fe2+ ions into Fe(OH)2."
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