${\mathrm{H}}_2\mathrm{S}$ gas when passed through a solution of cations containing HCl precipitates the cations of second group in qualitative analysis but not those belonging to the fourth group. It is because

In qualitative analysis of cations of second group, ${\mathrm{H}}_2\mathrm{S}$ gas is passed in the presence of HCl. Therefore, due to common ion effect, lower concentration of sulphide ions is obtained which is sufficient for the precipitation of second group cations in the form of their sulphides due to lower value of their solubility product $\left({\mathrm{K}}_{\mathrm{sp}}\right).$ Here fourth group cations are not precipitated because they require more sulphide ions for exceeding their ionic product to their solubility products which is not obtained here due to common ion effect.