JEE Advanced 2024 Paper 2 · Q17 · Stability & Applications
PARAGRAPH II: When potassium iodide is added to an aqueous solution of potassium ferricyanide, a reversible reaction is observed in which a complex P is formed. In a strong acidic medium, the equilibrium shifts completely towards P. Addition of zinc chloride to P in a slightly acidic medium results in a sparingly soluble complex Q.
The number of zinc ions present in the molecular formula of Q is _____.
Reveal answer + step-by-step solution
Correct answer:2, 3
Solution
Two valid stoichiometries for the zinc-ferrocyanide precipitate Q: (i) With limiting ZnCl$_2$: $3ZnCl_2 + 2K_4[Fe(CN)_6] \to K_2Zn_3[Fe(CN)_6]_2$ (white ppt) → 3 zinc ions per formula unit. (ii) With excess ZnCl$_2$: $2ZnCl_2 + K_4[Fe(CN)_6] \to Zn_2[Fe(CN)_6] + 4KCl$ (white ppt) → 2 zinc ions. The official key accepts both, hence answer is 2 or 3.
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