A solution containing a mixture of metal cations was treated with dilute HCI and
ID: 512912 • Letter: A
Question
A solution containing a mixture of metal cations was treated with dilute HCI and a precipitate formed. The solution was filtered and H_2S was bubbled through the acidic solution. A precipitate again formed and was filtered off. Then, the pH was raised to about 8 and H_2S was again bubbled through the solution. This time, no precipitate formed. Finally, the solution was treated with a sodium carbonate solution, which resulted in formation of a precipitate. Which metal ions were definitely present, which were definitely absent, and which may or may not have been present in the original mixture?Explanation / Answer
Cations of Group I form precipitates with HCl. The cations of group I are Pb2+, Hg22+ and Ag+.
Group II cations form precipitates with H2S in acidic medium. Sn2+, Sb3+, Cu2+ and Hg2+ belong to group II of the group separation table.
Group III cations form precipitates of sulfides with H2S or hydroxides in ammonical medium. Since no precipitate was obtained in alkaline medium, we can say that Group III cations like Zn2+, Ni2+ and Al3+ were absent.
Group IV cations form precipitates with sodium carbonate in neutral medium. Since a precipitate was obtained, group IV cations like Ba2+ was definitely present.
We didn’t conduct any test for Group V cations like K+ and Mg2+ and hence these cations may be present or may be absent.
Hence, we can fill up the chart as below:
Definitely present
Definitely absent
May or may not have been present
Pb2+, Hg22+, Ag+, Sn2+, Sb3+, Cu2+, Hg2+, Ba2+
Zn2+, Ni2+, Al3+
K+, Mg2+
Definitely present
Definitely absent
May or may not have been present
Pb2+, Hg22+, Ag+, Sn2+, Sb3+, Cu2+, Hg2+, Ba2+
Zn2+, Ni2+, Al3+
K+, Mg2+
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