A) increase the concentration of \[{{S}^{2-}}\]ions
B) limit the concentration of \[{{S}^{2-}}\]ions
C) add the \[C{{l}^{-}}\] ions
D) increase the solubility of \[{{H}_{2}}S\]
Correct Answer: B
Solution :
In the second group of qualitative analysis, dil. \[HCl\] is used before passing \[{{H}_{2}}S\] gas to limit the concentration of \[{{S}^{2-}}\] ions. \[{{H}_{2}}S2{{H}^{+}}+{{S}^{2-}}\] \[HCl\underset{common-ion}{\mathop{{{H}^{+}}}}\,\,\,+C{{l}^{-}}\] \[{{H}_{2}}S\] is a weak electrolyte but it can produce so much sulphide ion which are sufficient for the precipitation of sulphide of second group and other higher group mainly IVth group. But in the presence of \[HCl\] its ionisation will be decreased (due to common-ion) to such an extent that sulphide ion by it are sufficient for the precipitation of sulphide of second group and not other higher group.
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