Question

The formula mass of Mohr's salt is $392$. The iron present in it is oxidised by $KMn{O}_4$ in acidic medium. The equivalent mass of Mohr's salt is $ABC$, where $A,B$ and $C$ are numbers from $0$ to $9$. Which number does $B$ correspond to?

Answer 1

Mohr's salt is ammonium iron (II) sulphate, i.e., $\left(N{H}_4{)}_{2}Fe\right(S{O}_4{)}_2.6H_{2}O$.
During oxidation by $KMn{O}_4$, the following reaction occurs.
$F{e}^{2+}\to F{e}^{3+}+e^{-}$
Equivalent mass of Mohr's salt is given below: $\text{Equivalent mass}=\frac{\text{Molar mass of Mohr's salt}}{\text{Number of electrons involved in the oxidation half reaction}}=\frac{392}{1}=392$

It is given that the equivalent mass is equal to ABC. Hence, B corresponds to the number $9$.