The sodium salt X with warm dilute $H X_{2} SO X_{4}$ gives a pungent colourless gas that turns acidified dichromate paper green and a pale yellow turbidity. With excess $AgNO X_{3}$ , X gives a white precipitate that turns yellow, brown, and finally black. Atomic masses: S 32, O 16. For 0.100 mol of X reacting with excess acid, the salt and the volume of the gas at STP (22.4 L/mol) and mass of the turbidity are respectively

Answer: B

The turbidity (S) plus

SO X_{2}

and the white to black change of silver salt (

Ag X_{2} S X_{2} O X_{3} + H X_{2} O Ag X_{2} S + H X_{2} SO X_{4}

) identify thiosulphate.

Na X_{2} S X_{2} O X_{3} + H X_{2} SO X_{4} Na X_{2} SO X_{4} + S + SO X_{2} + H X_{2} O

gives 0.100 mol each of

SO X_{2}

and S. Volume = 0.100 x 22.4 = 2.24 L and mass of S = 0.100 x 32 = 3.2 g.