Analytical ChemistryHard

Question

H2S in the presence of HCl precipitates II group but not IV group because :

Options

A.HCl activates H2S
B.HCl increases concentration of Cl-
C.HCl decreases concentration of S2-
D.HCl lowers the solubility of H2S in solution

Solution

H2S ⇋ 2H+ + S2- ; HCl  ⇋ H2+ + Cl-.
Due to common ion effect, the ionisation of H2S is suppressed and thus low concentration of S2- ions is obtained. This much of S2- ions concentration is enough to precipitate only IInd group cations (because of the low KSP of IInd group sulphides).

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