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Which cation is NOT precipitated by H2S gas in either acidic medium (group II) or basic/ammoniacal medium (group IV) in qualitative analysis?

1. Pb²+
2. Zn²+
3. Ag⁺
4. Ba²+

Correct answer: Ba²+

Solution

Pb²+ is precipitated as PbS (black) in Group II (acidic H2S). Zn²+ is precipitated as ZnS (white) in Group IV (H2S in ammoniacal solution). Ag⁺ is typically precipitated in Group I (HCl as AgCl, white), and its sulphide Ag2S can also form in Group II. Ba²+ has a very soluble sulphide (BaS is quite soluble) and is NOT precipitated by H2S in either acidic or basic medium. Barium is separated in Group V using (NH4)2CO3. Hence Ba²+ is the answer.

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