JEE Main · 2022 · 26 Jul 2022 · Shift 1 · Redox Reactions and Electrochemistry

In $\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_4$ the four oxide ions carry a total charge of $-8$, so the three iron atoms together are in the $+8$ state. Reducing all of them to the metal, $\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_4+8e^{-}\to 3\mathrm{Fe}$, therefore requires $8$ faradays for every $3$ moles of iron produced. The charge needed for just one mole of iron is $\frac{8}{3}=2.67$ F, whose nearest integer is $3$.