$[X] + H_{2} S O_{4} \to [Y]$ , Y is a colorless gas with irritating smell.
$[Y] + K_{2} C r_{2} O_{7} + H_{2} S O_{4} \to$ Green solution ,with no coloured precipitate formation
X and Y are

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$S O_{3}^{2 -}, S O_{2}$

$C l^{-}, H C l$

$S^{2 -}, H_{2} S$

$C O_{3}^{2 -}, C O_{2}$

answer is A.

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Detailed Solution

Sulphite gives $S O_{2}$ with $H_{2} S O_{4}$ which turns acidified $K_{2} C r_{2} O_{7}$ green according to following reaction :
$K_{2} C r_{2} O_{7} + H_{2} S O_{4} + 3 S O_{2} \to K_{2} S O_{4} + C r_{2} {(S O_{4})}_{3} (g r e e n) + H_{2} O$ $H_{2} S$ also gives green coloured solution with acidified $K_{2} C r_{2} O_{7}$ solution but a slightly coloured precipitate of sulphur is also obtained. The $H_{2} S$ gas has rotten egg smell.