Question

In qualitative analysis when \[\ce{H2S}\] is passed through an aqueous solution of salt acidified with dil. \[\ce{HCl}\], a black precipitate is obtained. On boiling the precipitate with dil. \[\ce{HNO3}\], it forms a solution of blue colour. Addition of excess of aqueous solution of ammonia to this solution gives ______.

Options

• deep blue precipitate of \[\ce{Cu(OH)2}\]

• deep blue solution of \[\ce{[Cu (NH3)4]^{2+}}\]

• deep blue solution of \[\ce{Cu(NO3)2}\]

• deep blue solution of \[\ce{Cu(OH)2 .Cu(NO3)2}\]

Answer 1

In qualitative analysis when \[\ce{H2S}\] is passed through an aqueous solution of salt acidified with dil. \[\ce{HCl}\], a black precipitate is obtained. On boiling the precipitate with dil. \[\ce{HNO3}\], it forms a solution of blue colour. Addition of excess of aqueous solution of ammonia to this solution gives deep blue solution of \[\ce{[Cu (NH3)4]^{2+}}\].

Black precipitate of copper sulphide is formed which gives blue colour of copper nitrate on boiling with dilute \[\ce{HNO3}\]. When aqueous solution of ammonia is added to it, deep blue colour of

\[\ce{Cu^{2+} + H2S -> \underset{Black}{CuS} + 2H^{+}}\]

\[\ce{CuS + dil. HNO3 -> \underset{Blue}{Cu} (NO3)2}\]

\[\ce{Cu(NO3)2 + 4NH3 -> \underset{Deep blue}{[Cu(NH3)4]^{+}}}\]