Question

The formula mass of Mohr's salt is $392$. The iron present in it is oxidised by $KMn{O}_4$ in acid medium. The equivalent mass of Mohr's salt is:

• A. $392$
• B. $31.6$
• C. $278$
• D. $156$

Answer 1

The correct option is A $392$

Formula of Mohr's salt: $FeS{O}_4\cdot (N{H}_4{)}_{2}S{O}_4\cdot 6H_{2}O$
Oxidation of $F{e}^{2+}$ ions take place as:
$5F{e}^{2+}+Mn{O}_4^{-}+8H^{+}\to 5F{e}^{3+}+M{n}^{2+}+4H_{2}O$
Change in oxidation number of $Fe=+3-(+2)=+1$
Equivalent mass of Mohr's salt $=\frac{\text{Molecular mass}}{\text{Change in oxidation number}}$
$=\frac{392}{1}=392$.