The cation that will not be precipitated by \(\mathrm{H_2S}\) in the presence of dil HCl is:
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- \(\mathrm{Co^{2+}}\)
- \(\mathrm{As^{3+}}\)
- \(\mathrm{Pb^{2+}}\)
- \(\mathrm{Cu^{2+}}\)
The Correct Option is A
Solution and Explanation
\(\mathrm{Pb^{2+}}\) (PbS, black), \(\mathrm{Cu^{2+}}\) (CuS, black) and \(\mathrm{As^{3+}}\) (\(\mathrm{As_2S_3}\), yellow) are Group II cations and precipitate.
\(\mathrm{Co^{2+}}\) is a Group IV cation; CoS (larger \(K_{sp}\)) precipitates only in alkaline medium (\(\mathrm{NH_4OH}\) + \(\mathrm{NH_4Cl}\)) with \(\mathrm{H_2S}\).
Answer: (1) \(\mathrm{Co^{2+}}\)
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