A solution contains some or all of the ions Cu2+,Al3+, K+,Ca2+, Ba2+,Pb2+, and N

Question

A solution contains some or all of the ions Cu2+,Al3+, K+,Ca2+, Ba2+,Pb2+, and NH4+. The following tests were performed, in order, on the solution.

1.Addition of 6 M HCl produced no reaction.
2.Addition of H2S with 0.2 M HCl produced a black solid.
3.Addition of (NH4)2S produced a white solid.
4.Addition of (NH4)2HPO4 in NH3 produced no reaction.
5.The final supernatant when heated produced a purple flame.

Identify which of these ions were present in the solution (P), which were absent (A), and which were inconclusive (I)

a Cu2+

b Al3+

c K+

d Ca2+

e Ba2+

f Pb2+

g NH4+

Explanation / Answer

1.) 6M HCl(aq) no reaction. Pb2+ absent. All others maybe. 2.) H2S in H+ solution black precipitate. Cu2+ present. 3.) (NH4)2S white solid. Al3+b present 4) (NH4)2HPO4 + NH4OH no reaction. Ca+2, Ba2+ absent. You would conclude from this result alone that Cu2+ and Al3+ were absent, except that you filtered out their precipitates from earlier tests. 5.) This is not enough. You should be given a small length of platinum or nichrome wire with a tiny loop at one end. You should dip the loop is the 6M HCl solution and put it in a gas burner flame. There should be no color. Then dip the loop in the last filtrate from earlier tests and put it in the gas burner flame again. If it gives a lilac (not purple) flame, then potassium is present.

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