Question

The number of moles of $KMn{O}_4$ that will be needed to react with $1$ mol of sulphite ion in acidic solution is :

• A. $\frac{2}{5}$
• B. $\frac{3}{5}$
• C. $\frac{4}{5}$
• D. $1$

Answer 1

Answer: (A)

$\frac{2}{5}$

The redox changes are as follows:

$5e^{-}+Mn{O}_4^{⊝}\to M{n}^{2+}(n=5)$
$S{O}_3^{2-}\to S{O}_4^{2-}+2e^{-}(n=2)$
Equivalents of $Mn{O}_4^{⊝}=$ Equivalents of $S{O}_3^{2-}$
$\frac{1}{5}$ mol of $Mn{O}_4^{-}$ $=\frac{1}{2}$ mol of $S{O}_3^{2-}$
$\frac{2}{5}$ mol of $Mn{O}_4^{-}$ $=1$ mol of $S{O}_3^{2-}$

Hence, the number of moles of $Mn{O}_4$ that will be needed to react with $1$ mol of suphite in acidic solution is $\frac{2}{5}$.