Question

$KMn{O}_4$ acts as an oxidizing agent in acidic medium. The number of moles of $KMn{O}_4$ needed to react with one mole of sulphide ions in acidic solution is

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

Answer 1

The correct option is D $\frac{2}{5}$

Calculating required number of moles of $KMn{O}_4$
$Mn{O}_4^{-}$ ions get reduced to $M{n}^{2+}$ ions in acidic medium and sulphide ions get oxidised to sulphur.

Oxidation half reaction:
$5S^{2-}\to 5S+10e^{-}$

Reduction half reaction:
$2Mn{O}_4^{-}+10e^{-}+16H^{+}⟶2M{n}^{(}2+)+8H_{2}O$
Therefore, the net redox reaction involved is:

$2Mn{O}_4^{-}+16H^{+}+5S^{2-}$
$\downarrow$
$5S(\downarrow )+2M{n}^{2+}+8H_{2}O$
Therefore, $2$ moles of $Mn{O}_4^{-}$ are required to oxidise $5$ moles of $S^{2-}$ ions.
Therefore, to oxidise 1 mole of sulphide ions, $\frac{2}{5}$ moles of $Mn{O}_4^{-}$ ions are required.
So, option (A) is the correct answer.