JAMB - Chemistry (2003 - No. 34)
The salt that will form a precipitate soluble in excess ammonia solution is
Ca(NO3)2
Cu(NO3)2
Mg(NO3)2
Al(NO3)3
Explicació
When ammonia is added to a copper(II) nitrate solution, a pale blue precipitate of copper(II) hydroxide (Cu(OH)2) forms. The balanced equation for this reaction is
Cu(NO\(_3\))\(_2\) + 2NH\(_4\)OH→Cu(OH)2↓ + 2NH\(_4\)NO\(_3\)
Excess ammonia: When more ammonia is added, the precipitate dissolves and forms a deep blue solution of tetraammine copper(II) nitrate. The balanced equation for this reaction is Cu(OH)2(s)+4NH3(aq)−⇀↽−[Cu(NH3)4]+2(aq)+2OH−(aq)
This happens because the addition of more ammonia causes the formation of a coordination complex cation of copper(II) bonded to four ammonia molecules. This shifts the solubility equilibrium of copper(II) hydroxide to the right, causing the precipitate to dissolve.