Q.Which reagents are used to precipitate

Step 1. Qualitative-analysis group reagents are chosen to precipitate cations selectively, group by group, and the reagent used for one group is often deliberately different from the next so that only the intended group precipitates at a time.

Step 2. Group II cations (e.g. Cu2+, Cd2+, Pb2+ sulfides -- salts with very low Ksp) are precipitated as sulfides by passing H2S gas in the presence of dilute HCl; the acidic (H+-rich) medium suppresses the ionization of the weak acid H2S via the common ion effect, keeping [S2-] low -- but low enough [S2-] is still enough to exceed the very small Ksp values of these particular sulfides.

Step 3. Group III A cations (e.g. Fe3+, Al3+, Cr3+ hydroxides) are precipitated using NH4OH in the presence of NH4Cl -- an (NH4OH+NH4Cl) basic buffer that holds the pH at the 8-10 range needed to precipitate these hydroxides without also precipitating group IV cations. …

Recall the qualitative-analysis group-reagent scheme and connect each reagent choice to …

• Swapping group II and group III B reagents -- both use H2S but in OPPOSITE pH conditions (acidic vs ammoniacal), precisely to control [S2-] to match each group's Ksp range. …