An excess of Cl2 oxidises thiosulfate completely to sulfate, while I2 oxidises it only to tetrathionate, S4O6^2-. The ratio of moles of Cl2 to moles of I2 consumed per mole of S2O3^2- is

JEE Advanced levelSingle correct optionDifficulty 3 of 3ph044

An excess of $Cl X_{2}$ oxidises thiosulfate completely to sulfate, while $I X_{2}$ oxidises it only to tetrathionate, $S X_{4} O X_{6} X^{2 -}$ . The ratio of moles of $Cl X_{2}$ to moles of $I X_{2}$ consumed per mole of $S X_{2} O X_{3} X^{2 -}$ is

With

Cl X_{2}

: two S go from +2 to +6, 8 electrons lost, and each

Cl X_{2}

takes 2, so 4

Cl X_{2}

per thiosulfate. With

I X_{2}

2 S X_{2} O X_{3} X^{2 -} S X_{4} O X_{6} X^{2 -} + 2 e X^{-}

, so 1 electron per thiosulfate, i.e. 0.5

I X_{2}

. The ratio is 4 : 0.5 = 8 : 1.