Question

A solution contains the ions Ag⁺, Pb²⁺, and Ni²⁺. Dilute solutions of NaCl, Na₂SO₄, and Na₂S are available to separate the positive ions from each other. In order to effect separation, the solutions should be added in which order?

A) Na₂SO₄, NaCl, Na₂S
B) Na₂SO₄, Na₂S, NaCl
C) Na₂S, NaCl, Na₂SO₄
D) NaCl, Na₂S, Na₂SO₄
E) NaCl, Na₂SO₄, Na₂S

Answer 1

The ions Ag+, Pb2+, and Ni2+ can be separated by sequentially adding NaCl to precipitate AgCl, followed by Na2SO4 to precipitate PbSO4, and finally Na2S to precipitate NiS, using the principles of selective precipitation.

Step-by-step explanation:

To separate the positive ions Ag+, Pb2+, and Ni2+, we use the selective precipitation method. We need to add the solutions in a sequence that takes advantage of the different solubilities of the possible compounds formed. The correct order for the addition of the sodium salt solutions is:

1. NaCl: It forms insoluble AgCl which would precipitate, leaving Pb2+ and Ni2+ in solution.
2. Na2SO4: This precipitates PbSO4 since Pb2+ forms an insoluble sulfate but Ni2+ sulfate remains soluble.
3. Na2S: In the last step, adding sodium sulfide precipitates NiS as Ag+ and Pb2+ have been removed in the prior steps.

The sequence for the precipitation of ions is based on the solubility rules and the individual solubility products, which indicate the concentration at which a substance will precipitate from the solution. The intended result of following this sequential addition of reagents is to effectively separate all three cations into their respective precipitates.