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JEE Advanced 2024 · Paper 2, Chemistry question 16

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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 moles of potassium iodide required to produce two moles of P is _____.

Answer and worked solution

Official answer: 2

Iodide reduces ferricyanide to ferrocyanide. The reaction is reversible, and strong acid drives it completely to the right:
2 KX3[Fe(CN)X6]+2 KI⇌2 KX4[Fe(CN)X6]+IX2\ce{2K3[Fe(CN)6] + 2KI <=> 2K4[Fe(CN)6] + I2}

So P = KX4[Fe(CN)X6]\ce{K4[Fe(CN)6]}, and 2 mol of P need 2 mol of KI.

Topics:Stability and applicationsBalancing redox reactions
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