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A mixture of cations is treated with H2S gas in an acidic solution. Which group of cations will form insoluble sulfide precipitates under these conditions?
- Mn2+, Co2+, Zn2+
- Pb2+, Cu2+, Bi3+
- Ba2+, Sr2+, Ca2+
- Fe3+, Cr3+, Al3+
Correct answer: Pb2+, Cu2+, Bi3+
Solution
In acidic medium the concentration of S2- ions is low (H2S is a weak acid and H+ suppresses dissociation), so only cations with very low Ksp for their sulfides precipitate. Pb2+, Cu2+, and Bi3+ belong to Group II (HCl group sulfides) and form extremely insoluble sulfides (PbS, CuS, Bi2S3) even at low S2- concentration.
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