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.

Question: The number of moles of potassium iodide required to produce two moles of P is _____.

The correct answer is 3.

Let us write down the balanced chemical equation for the reaction of potassium iodide (KI) with potassium ferricyanide (K₃[Fe(CN)₆]) in an acidic medium.

In a strongly acidic medium, iodide ions (I^-) reduce hexacyanoferrate(III) ions ([Fe(CN)₆]^3-) to hexacyanoferrate(II) ions ([Fe(CN)₆]^4-), getting oxidized to iodine (I₂), which combines with excess iodide to form the triiodide complex anion (I₃^-), represented as complex P, K[I₃] or simply I₃^- in aqueous potassium iodide solution.

The overall ionic reaction for the formation of complex P (K[I₃]) can be written as:

$2{\left[\mathrm{Fe}{\left(\mathrm{CN}\right)}_6\right]}^{3-}+3{\text{I}}^{-}\leftrightarrows 2{\left[\mathrm{Fe}{\left(\mathrm{CN}\right)}_6\right]}^{4-}+{\text{I}}_3^{-}$

Here, the complex P formed is the triiodide complex species (or K[I₃]). From the balanced chemical stoichiometry:

1 mole of complex P (${\text{I}}_3^{-}$) is produced from 3 moles of iodide ions (KI).

Therefore, to produce 1 mole of P, 3 moles of KI are consumed, which corresponds to the given core value 3.