A solid sodium salt on treatment with dilute $H X_{2} SO X_{4}$ evolves a colourless gas with a pungent smell which turns acidified $K X_{2} Cr X_{2} O X_{7}$ paper green and leaves the solution clear even on standing. The gas from $0.252 g$ of the salt is passed through excess acidified dichromate. Using Na 23, S 32, K 39, Cr 52, O 16 and $Cr X_{2} O X_{7} X^{2 -} + 3 SO X_{2} + 2 H X^{+} 2 Cr X^{3 +} + 3 SO X_{4} X^{2 -} + H X_{2} O$ , the mass of $K X_{2} Cr X_{2} O X_{7}$ reduced is

Answer: C

No turbidity with dilute acid rules out thiosulphate, so the salt is

Na X_{2} SO X_{3}

(

M = 126

). Moles of

SO X_{2} = 0.252/126 = 0.00200

, and 3 mol

SO X_{2}

reduce 1 mol dichromate, giving

0.000667 mol

. Mass of

K X_{2} Cr X_{2} O X_{7}

(

M = 294

)

= 0.000667 \times 294 = 0.196 g