Sodium thiosulphate is used as an antichlor because chlorine oxidises thiosulphate completely to sulphate (as HSO4-) and itself goes to chloride. The number of moles of Cl2 consumed per mole of S2O3^2- is

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Sodium thiosulphate is used as an antichlor because chlorine oxidises thiosulphate completely to sulphate (as $HSO X_{4} X^{-}$ ) and itself goes to chloride. The number of moles of $Cl X_{2}$ consumed per mole of $S X_{2} O X_{3} X^{2 -}$ is

The average oxidation state of S rises from +2 to +6, so each S loses 4 electrons and the two S atoms lose 8 electrons. Each

Cl X_{2}

gains 2 electrons, so

8/2 = 4

mol

Cl X_{2}

are needed:

S X_{2} O X_{3} X^{2 -} + 4 Cl X_{2} + 5 H X_{2} O 2 HSO X_{4} X^{-} + 8 Cl X^{-} + 8 H X^{+}