A solution may contain Pb 2+ , Hg 2 2+ , and Ag + ions. A white precipitate form

Question

A solution may contain Pb2+, Hg22+, and Ag+ ions. A white precipitate formed upon the addition of 6M HCL solution. After heating, the solutuon was decanted from the solid A, The addition of ammonium carbonate to the decanted solution afforded a precipatate B. Solid A was then treated with 6M NH3 that yielded a colorless solutuon with no observed solid. Which ions are present, which are absent, or which may remain undetermined? Indicate the test(s) needed to verify the prescnce of the undetermined ion.

Explanation / Answer

Pb2+    +    2Cl-   ----> PbCl2 (s) white
Ag+    +       Cl-   ----> AgCl (s) white
Hg2 2+   +    2Cl-   ----> Hg2Cl2 (s) white

All 3 of them will form precipitate.
In presence of NH3:
AgCl + 2NH3 ---> Ag(NH3)2+ (Aq) colorless
and
Hg2Cl2 + 2NH3 ---> HgNH2Cl(s) + Hg(Aq) +NH4+ (aq) +Cl- (aq) dark grey precipitate
Since after reaction with NH3 solution is colorless
Ag+ is present in the solution and Hg2 2+ is absent.

Test for Pb2+:
It forms yellow precipitate with K2CrO4
Pb2+ + CrO42-    -----> PbCrO4 (s)    yellow

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