Question

A solution containing a mixture of metal cations was treated with dilute HCl 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? Ag+, Zn2+, Co2+, Li+, Cu2+, Ca2+, Fe2+, Sn2+, Mg2+, Hg22+ Sb3+, Hg2+, Pb2+

Answer 1

You can figure out this quesiton by figuring out which of these cations pair with the given anions to form insoluble precipitates. If you look up a solubility product chart, this will become much easier. I've provided a link below which is one such chart:

http://mak.ktf-split.hr/periodni/en/abc/kpt.html

Step 1) You added HCl, which is a source of the Cl^- anion. So now it's time to look at the chart and find out which of these metals form insoluble metal chloride salts: They are:

AgCl, Hg₂Cl₂ , PbCl₂

These cations could possibly present.

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Step 2) Adding H₂S to the solution in acidic and neutral media. The difference is that in acidic media, the group two metal cations can be precipitated but leave other metal cations in solution. When you raise the pH to neutral, then you can start precipitating others. Take a look at this link:

http://www.grossmont.edu/johnoakes/Ch142labs/142%20exp%2012%20cal%20poly%20group%20ii.pdf

So for the acidic addition of H₂S we get:

HgS, SnS₂ , Sb₂S₃ , CuS, PbS

So the precipitate that is formed after H₂S is bubbled through the acidic solution could be any of those metal cations.

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For the pH 8, the cations could be :

MnS, NiS, CoS, HgS (from the first chart), Ag₂S (also from the first chart)

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Step 3) Now you have CO₃^2- ions added into solution, which will pair with particular metal cations to form carbonate salts. Pull up the first chart and figure out which ones. This step will tell you which cations are definitely NOT in your solution, because no precipitate forms.

BaCO₃ , CoCO₃, PbCO₃ , Li₂CO₃? , MgCO₃? , MnCO₃? , Hg₂CO₃? , NiCO₃? , Ag₂CO₃? .

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So with all that information, we can cross-reference the ions we know are not there (from step 3) with the ions which could be there. This means we can't have Ba²⁺ , Co²⁺, Pb²⁺, Li⁺, Mg²⁺, Mn²⁺, Hg₂²⁺, Ni²⁺, or Ag⁺.

Referring to step 1, we see no precipitate forms. This makes sense, because the only metal cations which would form from treatment with HCl would be Hg₂²⁺, Ag⁺ and Pb²⁺ all of which we just eliminated as possibilities above.

Referring to step 2 with the acidic addition of H₂S, we see our possibilities are now only: HgS, SnS₂ , Sb₂S₃, CuS. Notice that PbS is no longer a possibility; it was the only species eliminates. This implies Hg²⁺, Sn⁴⁺, Sb³⁺ and Cu²⁺ may be present.

Referring to step 3 with the neutral addition of H₂S, we see our possibilities are now only: HgS. This implies Hg²⁺ is present.

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To summarize then, the possible present cations are Hg²⁺, Sn⁴⁺, Sb³⁺ and Cu²⁺.

The cations which are not possible are Ba²⁺ , Co²⁺, Pb²⁺, Li⁺, Mg²⁺, Mn²⁺, Hg₂²⁺, Ni²⁺, or Ag⁺.

I hope this makes sense and helps you!