A solution containing a mixture of metal cations was treated as follows 1. Dilut

Question

A solution containing a mixture of metal cations was treated as follows 1. Dilute HCI was added and a precipitate formed. The precipitate was filtered off. 2. H2S was bubbled through the acidic solution. Again, a precipitate formed and was filtered off. 3. The pH was raised to about 9 and H2S was again bubbled through the solution. No precipitate formed. 4. Finally, sodium carbonate was added and no precipitate formed. What can be said about the presence or each of these groups of cations in the origingl solution? Cation Group Description Present in the originalsolution? Ag', Pb2.Hg form insoluble chlorides BP CdP cu 2. form acid-insoluble Pb Sb3", sn2. AP Co?", cr Fe2 form base-insoluble Fes", Ni?". Mn sulfides or hydroxides Ba?", Ca?", Mg? Sr form insoluble carbonates Li Na K NH4. completely soluble Al of these ions were present None of these ions were present. At least one of these ions was present

Explanation / Answer

The question asks about the cathions for every group and how to identify them:

For group no.1 of cathions: HCl will make them precipitate in the form of chlorides. We can say that: At least one of these ions was present. (Ag, Pb, Hg2)

For group no.2 of cathions: The H2S in acidic to neutral conditions will make group 2 to precipitate. We can say that: At least one of these ions was present. (Bi, Cd, Cu, etc.)

For group no.3: The HsS in basic conditions will make group no.3 to precipite as sulfides or hidroxides. Since no precipitated was formed we can say that: None of these ions were present. (Al, Co, Cr, etc.)

For group no.4: The Carbonate will make group no. 4 to precipitate since no precipitated was form we can say that: None of these ions were present. (Ba, Ca, Mg, Sr)

The rest of the cathions remain in solution so: Li, Na, K, NH4+ , Certainly we cannot say if these cathios are present or not so. We can say that the presence of these cathions is unknown.

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