In qualitative analysis, when `H_2S` is passed through an aqueous solution of salt acidified with dil.HCl, a black precipitate is obtained. On boiling the precipitate with dil. `HNO_3`, it forms a solution of blue colour. Addition of excess of aqueous solution of ammonia to this solution gives

Options

A. deep blue precipitate of `Cu(OH)_2`
B. deep blue solution of `[Cu(NH_3)_4]^(2+)`
C. deep blue solution of `Cu(NO_3)_2`
D. deep blue solution of `Cu(OH)_2.Cu(NO_3)_2`

Answer

B

Explanation

A deep blue solution is formed due to the following reactions :
`Cu^(2+) + H_(2)S overset(dil HCl) rarr underset("(Black ppt.)")(CuS) + 2H^(+) , " " CuS + underset("(dil)")(2NHO_3) rarr underset("(Blue solution)")(Cu(NO_3)_2) + H_2S`
`Cu(NO_3)_2 + 4NH_3 rarr underset("(Deep blue solution)")([Cu(NH_3)_4]^(2+)) + 2NO_(3)^(-)`