Question

$[X] + dil , H_2SO_4 \rightarrow [Y]$ : Colourless, suffocating gas
$[Y] + K_2 Cr_2 O_7 + H_2 SO_4 \rightarrow$ Green colouration of solution Then, [X] and [Y] are

• A. $SO^{2-}_{3} , SO_2$
• B. $Cl^{-},HCl$
• C. $S^{2-} , H_2 S$
• D. $CO^{2-}_{3}, CO_2$

Answer 1

Answer: (A)

According to question,

$[X]+$ Dil. $H _{2} SO _{4} \longrightarrow [Y]:$

Colourless, suffocating gas

$[Y]+ K _{2} Cr _{2} O _{7}+ H _{2} SO _{4} \longrightarrow $

Green colouration of solution.

All sulphites when treated with dil. $H _{2} SO _{4}$ gives colourless and suffocating sulphur dioxide gas.

$\underset{\overset{\text{Sulphite ion}}{[X]}}{SO_3^{2-}(s)} +$ Dil. $H_2SO_4(aq) \rightarrow SO_4^{2-}(aq)$
$ + \underset{\overset{\text{Sulphur}}{\text{dioxide[Y]}}}{SO_2(g)} + H_2O(l)$

$SO _{2}$ gas $[Y]$ turns acidified potassium dichromate paper green due to reduction of $Cr ( VI )$ to $Cr ( III )$.

$K _{2} Cr _{2} O _{7}(a q)+ H _{2} SO _{4}(a q)+ \underset{[Y]}{3 SO _{2}(g)} \longrightarrow $
$K _{2} SO _{4}(a q)+\underset{\text { Green }}{ Cr _{2}\left( SO _{4}\right)_{3}(a q)}+ H _{2} O (l)$