Question

In presence of $HCl,H_{2}S$ results the precipitation of Group-2 elements but not Gp-4 elements during qualitative analysis. It is due to

• A. higher concentration of $S^{2-}$
• B. higher concentration of $H^{+}$
• C. lower concentration of $S^{2-}$
• D. lower concentration of $H^{+}$

Answer 1

Answer: (C)

Sulphides of group-II are precipitated by passing $H_{2}S$ gas through the solutions of cations in the presence of dilute HCl.

$H_{2}S$ being a weak electrolyte ionises only slightly while HCl being a strong electrolyte is almost completely ionised.

$H_{2}S\rightleftharpoons 2H+S^{2-}$ . $HCl\to H^{+}+C{l}^{-}$

Thus, the common ion effect takes place. As a result, the degree of dissociation of $H_{2}S$ decreases sufficiently and the concentration of $S^{2-}$ ions in the solution becomes very small, which results in the precipitation of those sulphides having very low solubility product.