Question

When ammonium hydroxide is added to solution B, a pale blue precipitate is formed. This pale blue precipitate dissolves in excess ammonium hydroxide giving an inky blue solution. What is the cation [positive ion] present in solution B? what is the probable colour of solution B.

Answer 1

\[\ce{CuSO4 + 2NH4OH -> (NH4)SO4 + \underset{[Pale Blue ppt]}{Cu(OH)2}}\]

The cation present in solution B is Copper (Cu⁺²).

The probable colour of solution B is blue.

The pale blue precipitate of copper hydroxide dissolves in excess of ammonium hydroxide, forming tetraamine copper [II] sulphate, an azure blue (deep blue) soluble complex salt.

\[\ce{\underset{[Pale blue ppt]}{Cu[OH]2} + \underset{[Amm. Hydroxide in excess]}{(NH3)2SO4} + 2NH4OH -> \underset{\underset{(Deep blue colouration) or Azure blue sol}{[Tetra amine copper II sulphate]}}{[Cu(NH3)4]SO4} + 4H2O}\]