Question

An unknown solution may contain any of the seven cations listed below. One student preformed a series of experiments to determine what ions are in the solution. Based upon the experiments performed by the students, the results are as follows:

On addition of 6 M HCl, a white precipitate results. The white precipitate is then mixed with NH3 where it dissolves. b) When acidic H2S is added to the unknown, a black precipitate results, and is dissolved in NH3 giving a deep blue color. c) When ammonium carbonate, concentrated ammonia, and ammonium chloride are added to the unknown, a clear solution results.

Silver:

Aluminum:

Barium:

Answer 1

A. On adding HCl to the unknown solution group 1 cations precipitate out. Out of all the group 1 cations Ag forms white precipitate of AgCl with HCl which solubalises in ammonia solution as shown.

Ag⁺ + Cl^- AgCl (white)

AgCl + NH₄OH Ag(NH₃)₂⁺ (soluble)

B. Group 2 cations are precipitated using acidified H₂S. Out of all the group 2 cations, Cu forms a black precipitate which on addition of ammonia changes to blue.

Cu²⁺ + H₂S CuS (black)

CuS + NH₃ deep blue colour

C. NH₄Cl , NH₄OH and (NH₄)CO₃ are used to precipitate group 4 cations i.e. Ba²⁺, Ca²⁺ and Sr²⁺. A clear solution indicates these ions are absent.