Moles of Mohr's Salt Required per Mole of Dichromate

In acidic medium, the dichromate ion ($\text{Cr}_2\text{O}_7^{2-}$) acts as a strong oxidizing agent and is reduced to $\text{Cr}^{3+}$:
$\text{Cr}_2\text{O}_7^{2-} + 14\text{H}^+ + 6e^- \rightarrow 2\text{Cr}^{3+} + 7\text{H}_2\text{O}$

Mohr's salt, $\text{FeSO}_4\cdot(\text{NH}_4)_2\text{SO}_4\cdot 6\text{H}_2\text{O}$, contains $\text{Fe}^{2+}$ which is oxidized to $\text{Fe}^{3+}$:
$\text{Fe}^{2+} \rightarrow \text{Fe}^{3+} + e^-$

Multiplying the iron half-reaction by 6 and adding:
$\text{Cr}_2\text{O}_7^{2-} + 14\text{H}^+ + 6\text{Fe}^{2+} \rightarrow 2\text{Cr}^{3+} + 6\text{Fe}^{3+} + 7\text{H}_2\text{O}$

Thus, $1\text{ mole}$ of potassium dichromate requires exactly 6 moles of Mohr's salt.

Correct Answer: D