Chemical Formula of Precipitate X

To determine the correct chemical formula for precipitate $\mathbf{X}$, we analyze the qualitative test for iron ions in aqueous solution:

1. Reaction Mechanism: When ferric ions ($\text{Fe}^{3+}$) react with potassium ferrocyanide ($\text{K}_4[\text{Fe}(\text{CN})_6]$), a characteristic Prussian blue precipitate is formed.

2. Chemical Equation: The balanced ionic equation for the formation of the precipitate is: $4\text{Fe}^{3+}(aq) + 3[\text{Fe}(\text{CN})_6]^{4-}(aq) \longrightarrow \text{Fe}_4[\text{Fe}(\text{CN})_6]_3(s)$

3. Oxidation State Analysis:

   • The iron outside the coordination sphere is in the $+3$ oxidation state ($\text{Fe}^{3+}$).
   • The iron inside the hexacyanoferrate complex ion is in the $+2$ oxidation state ($\text{Fe}^{2+}$).
   • Balancing total charge: $4(+3) + 3[+2 + 6(-1)] = 12 + 3(-4) = 0$

Thus, the formula of the insoluble Prussian blue precipitate $\mathbf{X}$ is $\text{Fe}_4[\text{Fe}(\text{CN})_6]_3$.

Correct Option: (A)