The group II reagent in the cation analysis is the dil.HCl + H₂S. The role of dil.HCl is

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To decrease $[S^{2 -}]$ and so increase the ionic product to exceed the $K_{Sp}$ of group IV sulphides and prevent their precipitation.

To dissolve group II metal sulphides in acid medium

To decrease $[S^{2 -}]$ concentration so that the ionic product of only group II sulphides exceed their K_sp values

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Detailed Solution

$H_{2} S ⇌ 2 H^{+} + S^{2 -}, HCl \to H^{+} + {Cl}^{-}$
Due to common ion effect $[H^{+}] [S^{2 -}]$ decreases. $K_{Sp}$ values of Group-II sulphides are less than Group-IV sulphides. The decreased concentration is sufficient to precipitate Gr-II cations, but not Group-IV cations.