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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
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Verified Answer
The correct answer is:
$\mathrm{Na}_{2} \mathrm{~S}_{2} \mathrm{O}_{3}$
Hint:
$\underset{\text { (colouriess, efflorescent) }}{\mathrm{Na}_{2} \mathrm{~S}_{2} \mathrm{O}_{3}(\mathrm{~s})}+2 \mathrm{H}^{+}(\mathrm{dil} .) \rightarrow \mathrm{S}(\mathrm{s})+2 \mathrm{Na}^{+}(\mathrm{aq})+\mathrm{SO}_{2} \uparrow+\mathrm{H}_{2} \mathrm{O}(\ell)$
$\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}($ aq. $)+3 \mathrm{SO}_{2}(\mathrm{~g})+2 \mathrm{H}^{+}(\mathrm{aq} .) \longrightarrow 2 \mathrm{~K}^{+}($ aq. $)+\underset{(\text { green })}{2 \mathrm{Cr}^{+3}}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\ell)+3 \mathrm{SO}_{4}^{-2}(\mathrm{aq})$
$\underset{\text { (colouriess, efflorescent) }}{\mathrm{Na}_{2} \mathrm{~S}_{2} \mathrm{O}_{3}(\mathrm{~s})}+2 \mathrm{H}^{+}(\mathrm{dil} .) \rightarrow \mathrm{S}(\mathrm{s})+2 \mathrm{Na}^{+}(\mathrm{aq})+\mathrm{SO}_{2} \uparrow+\mathrm{H}_{2} \mathrm{O}(\ell)$
$\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}($ aq. $)+3 \mathrm{SO}_{2}(\mathrm{~g})+2 \mathrm{H}^{+}(\mathrm{aq} .) \longrightarrow 2 \mathrm{~K}^{+}($ aq. $)+\underset{(\text { green })}{2 \mathrm{Cr}^{+3}}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\ell)+3 \mathrm{SO}_{4}^{-2}(\mathrm{aq})$
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