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  4. H2S 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

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Q. $ 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

AIPMTAIPMT 2005Equilibrium

Solution:

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.