$\mathrm{H}_2 \mathrm{~S}$ gas when passed through a solution of cations containing $\mathrm{HCl}$…
$\mathrm{H}_2 \mathrm{~S}$ gas when passed through a solution of cations containing $\mathrm{HCl}$ precipitates the cations of second group of qualitative analysis but not those belonging to the fourth group. It is because
presence of $\mathrm{HCl}$ decreases the sulphide ion concentration
solubility product of group II sulphides is more than that of group IV sulphides
presence of $\mathrm{HCl}$ increases the sulphide ion concentration
sulphide of group IV cations are unstable in $\mathrm{HCl}$
Solution
In qualitative analysis of cation of second group, $\mathrm{H}_2 \mathrm{~S}$ gas is passed in presence of $\mathrm{HCl}$, sulphide ions obtained is sufficient for the precipitation of second group cations in form of their sulphides due to lower value of their solubility product $\left(\mathrm{K}_{\mathrm{sp}}\right)$. Hence, fourth group cations are not precipitated because they require for exceeding their ionic product to their solubility products and higher sulphide ions concentration due to their higher $\mathrm{K}_{\mathrm{sp}}$ which is not obtained here due to common ion effect.
common ion
Caution
$\mathrm{HCl}$ is added before $\mathrm{H}_2 \mathrm{~S}$ gas during qualitative analysis of second group radicals because the concentration of sulphide ions is suppressed by $\mathrm{HCl}$ due to common ion effect. The already present $\mathrm{H}^{+}$ions suppress the dissociation of $\mathrm{H}_2 \mathrm{~S}$ by moving the reaction in the backward reaction.