Thiosulphate reacts with iodine to give tetrathionate, but with excess chlorine water it is oxidised completely to sulphate. 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

Answer: B

With iodine:

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

so 0.5 mol

I X_{2}

per mole. With chlorine: S (average +2) goes to +6, i.e. 8 electrons per

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

, hence 4 mol

Cl X_{2}

. Ratio = 4 : 0.5 = 8 : 1.