Question

In presence of $HCl, H_2S$ 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)

lower concentration of $S^{2-}$

Answer 2

Sulphides of group-II are precipitated by passing $H_2S$ gas through the solutions of cations in the presence of dilute HCl.

$H_2S$ being a weak electrolyte ionises only slightly while HCl being a strong electrolyte is almost completely ionised.

${H2S <=> 2H + S^{2-}}$ . ${HCl ->H+ + Cl-}$

Thus, the common ion effect takes place. As a result, the degree of dissociation of H2S 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.