Question

Which of them does not react with aqueous solution of $KMnO_4$ in presence of $H_2SO_4$ ?

• A. $NO^-_3$
• B. $S^{2-}$
• C. $I^-$
• D. $NO^-_2$

Answer 1

Answer: (A)

$KMnO_4$ oxidises
$H_2S(S^{2-})$ to $S$
$2MnO_4^- +16H^+ +5s^{2-} \to 2Mn^{2+} + 8H_2O + 5s$
$KI (I^-) $ to $I_2$
$2MnO_{4}^{-} +16H^{+} + 10 I^- \to 2Mn^{2+} + 8H_2O +5I_2$
$KNO_2(NO_2)$ to $NO_3^-$
$2MNO_4^- +6H^+ +5NO_2^- \to 2Mn^{2+} +3H_2O +5NO_3^-$
In $NO_3^-, N$ is present in its highest oxidation state $(+5)$, so it cannot be oxidised further.