Qualitative Analysis - Result Question 25
####25. The reagent(s) that can selectively precipitate $S^{2-}$ from a mixture of $S^{2-}$ and $SO _4^{2-}$ in aqueous solution is (are)
(2016 Adv.)
(a) $CuCl _2$
(b) $BaCl _2$
(c) $Pb\left(OOCCH _3\right) _2$
(d) $Na _2\left[Fe(CN) _5 NO\right]$
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Solution:
- $S^{2-}+CuCl _2 \longrightarrow CuS \downarrow$ (black ppt.)
$$ SO _4^{2-}+CuCl _2 \longrightarrow \text { Soluble, Thus } $$
(a) $CuCl _2$ selectively precipitates $S^{2-}$.
(b) $S^{2-}+BaCl _2 \longrightarrow BaS \downarrow$ (soluble)
$$ SO _4^{2-}+BaCl _2 \longrightarrow BaSO _4 \downarrow \text { (white ppt.) } $$
(b) precipitates $SO _4^{2-}$ and not $S^{2-}$.
(c) $S^{2-}+Pb^{2+} \longrightarrow PbS \downarrow$ (black ppt.)
$SO _4^{2-}+Pb^{2+} \longrightarrow PbSO _4 \downarrow$ (white ppt.)
$S^{2-}$ and $SO _4^{2-}$, both are precipitated.
But no colour with $SO _4^{2-}$.
