Q.vii. Sulfides of cation of group II are precipitated in acidic solution ($\mathrm{H_2S}$ + HCl) whereas sulfides of cations of group IIIB are precipitated in ammoniacal solution of $\mathrm{H_2S}$. Comment on the relative values of solubility product of sulfides of these.

Step 1. Recall the reagents. Group II cation sulfides are precipitated using H2S in the presence of dilute HCl (acidic medium); group IIIB cation sulfides are precipitated using H2S in an ammoniacal (basic) medium -- (the 'Can you recall?' box of section 3.10).

Step 2. Connect medium to [S2-]. H2S is itself a weak diprotic acid, $H_2S\rightleftharpoons2H^++S^{2-}$ (overall). In an ACIDIC medium, the extra H+ from HCl is a common ion with H2S's own ionization products, so by the common ion effect (section 3.10) H2S's ionization is strongly suppressed, keeping [S2-] LOW. In a BASIC (ammoniacal) medium, there is no such H+ suppression -- indeed the OH- present consumes H+ as it forms -- so H2S ionizes much more fully, giving a much HIGHER [S2-].

Step 3. Apply the precipitation condition. From section 3.9.3, a sulfide MX precipitates once $IP=[M^{n+}][S^{2-}]>K_{sp}$. For a given [M(n+)], precipitation requires [S2-] to be at least $K_{sp}/[M^{n+}]$. …

Connect the two different precipitation media to the resulting [S2-] (via the common ion effect on H2S's ionization), then use the IP>Ksp precipitati …

• Assuming both groups have similar Ksp and that the medium alone explains the separation -- the medium controls [S2-], but the SEPARATION only works because the two groups' Ksp values genuinely differ. …