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 zinc ions present in the molecular formula of Q is _____.

The correct answer is 2.

Step-by-Step Explanation:

Step 1: Reaction of Potassium Ferricyanide with Potassium Iodide
When potassium iodide ($KI$) is added to an aqueous solution of potassium ferricyanide ($K_3\left[Fe\right(CN{)}_6]$), a redox equilibrium is established:
$2\left[Fe\right(CN{)}_6{]}^{3-}+2I^{-}\rightleftharpoons 2\left[Fe\right(CN{)}_6{]}^{4-}+I_2$
In a strongly acidic medium, the equilibrium shifts completely to the right, forming the ferrocyanide complex ion, $\left[Fe\right(CN{)}_6{]}^{4-}$ (which corresponds to complex P, i.e., potassium ferrocyanide $K_4\left[Fe\right(CN{)}_6]$).

Step 2: Addition of Zinc Chloride to Complex P
When zinc chloride ($ZnC{l}_2$) is added to potassium ferrocyanide in a slightly acidic medium, a characteristic bluish-white sparingly soluble complex precipitate Q is formed:
$2ZnC{l}_2+K_4\left[Fe\right(CN{)}_6]\to Z{n}_2[Fe\left(CN{)}_6\right]+4KCl$
(or as a mixed salt $K{n}_{3}Z{n}_3\left[Fe\right(CN{)}_6{]}_2$, but standard zinc ferrocyanide is represented as $Z{n}_2\left[Fe\right(CN{)}_6]$).

Step 3: Determining the Number of Zinc Ions
The chemical formula of complex Q is $Z{n}_2\left[Fe\right(CN{)}_6]$.
Therefore, the number of zinc ions ($Z{n}^{2+}$) present in the molecular formula of Q is 2.