Chlorine oxidises thiosulphate to sulphate (Na2S2O3 + 4Cl2 + 5H2O -> 2NaHSO4 + 8HCl), whereas iodine oxidises it to tetrathionate (2S2O3^2- + I2 -> S4O6^2- + 2I-). The ratio of moles of Cl2 to moles of I2 needed to react with the same amount of thiosulphate is

JEE Advanced levelSingle correct optionDifficulty 3 of 3ph197

Chlorine oxidises thiosulphate to sulphate ( $Na X_{2} S X_{2} O X_{3} + 4 Cl X_{2} + 5 H X_{2} O 2 NaHSO X_{4} + 8 HCl$ ), whereas iodine oxidises it to tetrathionate ( $2 S X_{2} O X_{3} X^{2 -} + I X_{2} S X_{4} O X_{6} X^{2 -} + 2 I X^{-}$ ). The ratio of moles of $Cl X_{2}$ to moles of $I X_{2}$ needed to react with the same amount of thiosulphate is

Per mole of thiosulphate,

Cl X_{2}

needed is 4 mol. Per mole of thiosulphate,

I X_{2}

needed is

\frac{1}{2}

mol, since 2

S X_{2} O X_{3} X^{2 -}

react with one

I X_{2}

. The ratio is

4/0.5 = 8

. This is why chlorine is used as an antichlor.