Question

\[{H_2}S\;\] gas when passed through a solution of cations containing\[\;HCl\] precipitates the cations of second group of qualitative analysis but not those belonging to the fourth group it is because
(A) Presence of \[\;HCl\] decreases the sulphide ion concentration
(B) Solubility product of group $II$sulphates is more than that of group $IV$ sulphides
(C) Presence of \[\;HCl\] increases the sulphide ion concentration
(D) Sulphides of group $IV$cations are unstable in \[\;HCl\]

Answer 1

Hint: Second group of cations are precipitated on passing \[{H_2}S\;\] gas in an acidic medium say\[\;HCl\]. While the fourth group cations are precipitated in presence of basic medium. These groups are classified on the basis of selective precipitation of sparingly soluble salts.

Complete Step by Step Solution:
Hydrogen sulphide is a weak acid which dissociates as the following reaction
${H_2}S\underset {} \leftrightarrows {H^ + } + {S^{2 - }}$
As it is a weak acid so the reaction is in both directions.

Dissociation of \[\;HCl\]can be considered as follow:
$HCl\xrightarrow{{}}{H^ + } + C{l^ - }$
\[\;HCl\]is a strong acid hence is largely ionized to give ${H^ + }$ions. Thus, concentration of hydronium ions is increased and consequently the concentration of sulphide ions produced by ionisation of \[{H_2}S\;\]is sufficiently decreased due to common ion effects. As a result of which the sulphide ion concentration is sufficient enough to exceed the solubility product of sulphides of group $2$ cations. Since K_sp of sulphides of group $3$ and $4$cations is very high so these cations are not precipitated.

Thus, it is a common ion effect that suppresses the dissociation of\[{H_2}S\;\]to achieve precipitation conditions. This is due to the presence of $HCl$ that decreases the sulphide ion concentration.
Hence, the correct answer is A.

Note: Any other acid cannot be used in place of \[\;HCl\]. Weak acids like acetic acid are themselves partially dissociating that cannot suppress the dissociation of \[{H_2}S\;\]. Sulphuric acid will lead to precipitation of sulphates of group $5$. Nitric acid is a powerful oxidising agent that will oxidise \[{H_2}S\;\]to sulphur. Even concentrated hydrochloric acid cannot be used because it will suppress dissociation of \[{H_2}S\;\]to a large extent.