A solution containing a mixture of metal cations was treated with dilute HCI and
ID: 996859 • 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 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?Explanation / Answer
For a better understanding of this question, it is advisable to go through separation of cations by specific group reagents. The cations are divided into 5 groups on the basic of their specificity toward specific group reagents.
1) Group I cations form precipitates with dilute HCl. The common examples of group I cations are Pb2+, Hg+ and Ag+.
2) Group II cations form precipitates with H2S in acidic medium. These cations do not react with dilute HCl. Common examples are Hg2+, Cu2+, Bi3+, Cd2+, Sn2+, Sn4+, As3+, As5+, Sb3+ and Sb5+.
3) Group III cations do not react with dilute HCl or H2S but precipitate sulphides with (NH4)2S in ammoniacal medium. Examples are Fe2+, Fe3+, Co2+, Ni2+, Mn2+, Cr3+, Al3+ and Zn2+.
4) Group IV cations do form react with any of the reagents listed above, but instead precipitate carbonates with (NH4)2CO3 in presence of ammonium chloride in neutral medium. Ca2+, Ba2+ and Sr2+ belong to this group.
5) Group V cations do not react with any of the group reagents listed above. The ions in this group are Mg2+, Na+, K+ and NH4+.
In the present case, a precipitate with dilute HCl was formed. Hence, Pb2+ and Ag+ are definitely present. Hg22+ may be present.
A precipitate was formed with H2S in dilute acid medium. Hence, Cd2+, Bi3+, Hg2+ and Sn2+ are definitely present.
No precipitate with hydrogen sulphide was formed when the pH was raised to 8. Group III cations form precipitates in neutral to ammoniacal medium. Hence, Fe2+, Cr3+ and Zn2+ are mosty likely absent.
A precipitate was formed with sodium carbonate. Ba2+ is definitely present.
Mg2+ and K+ may or may not be present.
Definitely Present: Pb2+, Ag+, Cd2+, Bi3+, Hg2+, Sn2+, Ba2+
Definitely Absent: Fe2+, Cr3+, Zn2+
May be present: Hg22+, Mg2+, K+
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