Question

$H_2S$ 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

• A. presence of HCI decreases the sulphide ion concentration
• B. presence of HCI increases the sulphide ion concentration
• C. solubility product of group II sulphides is more than that of group IV sulphides
• D. sulphides of group IV cations are unstable in HCI

Answer 1

Answer: (A)

presence of HCI decreases the sulphide ion concentration

Answer 2

The correct answer is A:presence of $HCl$ decreases the sulphide ion concentration
In qualitative analysis of cations of second group $H_2S$ gas is passed in 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 $(K_{sp})$. Here, fourth group cations are not precipitated because it require more sulphide ions for exceeding their ionic product to their solubility products which is not obtained here due to common ion effect.