. H2 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 sulphides 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.


Best Answer

Correct Option is  (a) : 

The sulphides of group II cations are precipitated.The sulphide product of group II radicals has a relatively poor solubility. Therefore, even with modest conc. of S ions, the ionic product surpasses the value of their solubility product and the radicals of group II gets precipitated. Passing H 2 S gas through a salt solution in the presence of dil. HCl lowers the degree of ionisation of H 2 S through the common ion effect, resulting in a low concentration of S2 ions. It's worth noting that group IV radicals have a high solubility product. The conc. of S(2-) ions must be suppressed, or else radicals from group IV will precipitate alongside group II radicals.


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