The gas formed from sodium sulphite and dilute H2SO4 turns lime water milky and decolourises acidified KMnO4: 2MnO4^- + 5SO2 + 2H2O -> 2Mn^2+ + 5SO4^2- + 4H+. The number of moles of KMnO4 decolourised by 1 mol of this gas is

JEE Main levelSingle correct optionDifficulty 2 of 3qa029

The gas formed from sodium sulphite and dilute $H X_{2} SO X_{4}$ turns lime water milky and decolourises acidified $KMnO X_{4}$ : $2 MnO X_{4} X^{-} + 5 SO X_{2} + 2 H X_{2} O 2 Mn X^{2 +} + 5 SO X_{4} X^{2 -} + 4 H X^{+}$ . The number of moles of $KMnO X_{4}$ decolourised by 1 mol of this gas is

The balanced equation shows 2 mol

MnO X_{4} X^{-}

react with 5 mol

SO X_{2}

. So 1 mol

SO X_{2}

needs 2/5 = 0.4 mol of

KMnO X_{4}

. The inverse, 2.5, is the common error.