Question

When $H_{2}S$ gas is passed through a hot acidic aqueous solution containing $C{r}^{3+},C{d}^{2+},H{g}^{2+}andC{o}^{2+},$ a precipitate is formed which does not consist of :

• A. $CdSandHgS$
• B. $CdSandCoS$
• C. $C{r}_2{S}_{3}andHgS$
• D. $CoSandC{r}_2{S}_3$

Answer 1

The correct option is D $CoSandC{r}_2{S}_3$

$H_{2}S$ gas in acidic medium is the reagent for group II cation.
Here,
$C{r}^{3+}$ is of group III and $C{o}^{2+}$ is of group IV
$C{d}^{2+}andH{g}^{2+}$ are of group II
So, $CdS$ and $HgS$ will be fomed, whereas $C{r}_2{S}_3$ and $CoS$ will not be formed.
Group III form hydroxides whose reagent is $N{H}_{4}OH$ in the presence of $N{H}_{4}Cl$. Reagent for ggroup IV is $H_{2}S$ in basic media.
Option (a) is correct.