A solution containing a mixture of metal cations was treated with dilute HCI and
ID: 1031358 • 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 H2S was bubbled through the acidic solution. A precipitate again formed and was filtered off. Then, the pH was raised to about 8 and H2S 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? Definitely absent Possibly present 2+ 2+ 2+ 2+ 2+Explanation / Answer
1) Group I cations give a precipitate with dilute HCl. The group I cations are Pb2+, Hg22+ and Ag+.
2) The solution from step (1) was filtered and treated with H2S in dilute HCl to get a precipitate. Group II cations give a precipitate with H2S in acidic medium. The group II cations include Pb2+, Hg2+, Cu2+, Sn4+ and Bi3+.
3) The solution from step (2) was filtered and made alkaline (it is said that the pH was raised to 8) and H2S was bubbled through the solution. No precipitate was formed which indicates that group III cations were absent. The group III cations are Zn2+, Fe2+ and Cr3+.
4) The solution was step (3) was filtered and treated with sodium carbonate which gave a precipitate. Therefore, one or more group IV cations were definitely present. Ba2+ is the only group IV cation present in the list and hence, was definitely present.
5) No tests were performed for group V cations and hence the group V cations, K+ and Mg2+ may or may not have been present.
Definitely Present
Definitely Absent
Possibly Present
Ba2+
Zn2+, Fe2+ or Cr3+
Pb2+, Hg22+, Ag+, Hg2+, Cu2+, Sn4+, Bi3+, K+, Mg2+
Definitely Present
Definitely Absent
Possibly Present
Ba2+
Zn2+, Fe2+ or Cr3+
Pb2+, Hg22+, Ag+, Hg2+, Cu2+, Sn4+, Bi3+, K+, Mg2+
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