Question

To a solution of a colourless efflorescent sodium salt, when dilute acid is added, a colourless gas is evolved along with formation of a white precipitate. Acidified dichromate solution turns green when the colourless gas is passed through it. The sodium salt is
(A) \(\mathrm{Na}_{2} \mathrm{SO}_{3}\)
(B) \(\mathrm{Na}_{2} \mathrm{~S}\)
(C) \(\mathrm{Na}_{2} \mathrm{~S}_{2} \mathrm{O}_{3}\)
(D) \(\mathrm{Na}_{2} \mathrm{~S}_{4} \mathrm{O}_{6}\)

Answer 1

Ans: (C)
 \(\underset{\text { (colourless,efflorescent) }}{\mathrm{Na}_{2} \mathrm{~S}_{2} \mathrm{O}_{3}(\mathrm{~s})}+2 \mathrm{H}^{+}(\)dil. \() \rightarrow \underset{\text { (white ppt) }}{\mathrm{S}(\mathrm{s})}+2 \mathrm{Na}^{+}(\)aq. \()+\mathrm{SO}_{2} \uparrow+\mathrm{H}_{2} \mathrm{O}(\ell)\)
\(\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}\) (aq.) \(+3 \mathrm{SO}_{2}(\mathrm{~g})+2 \mathrm{H}^{+}(\)aq. \() \longrightarrow 2 \mathrm{~K}^{+}(\mathrm{aq})+2 \mathrm{Cr}^{+3}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\ell)+3 \mathrm{SO}_{4}^{-2}(\mathrm{aq})\)
(green)

Answer 2

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