In qualitative analysis when `H_(2)S` 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

In qualitative analysis when `H_(2)S` is passed through an aqueous solution of salt acidified with dil. HCl a black ppt. of CuS is obtained.
`CuSO_(4)+H_2O overset("dil. HCl")to underset("black ppt")CuS+H_(2)SO_(4)`
On boiling CuS with dil. `HNO_(3)` it forms a blue coloured solution and the following reactions occur
`3CuS+8HNO_(3) to 3Cu(NO_(3))_(2)+2NO+3S+4H_(2)O`
`S+2HNO_(3) to H_(2)SO_(4)+NO`
`2CU^(2+)+SO_(4)^(2+) +2NH_(3)+2H_(2)O to CU(OH)_(2).CuSO_(4) +2NH_(4)OH`
`Cu(OH)_(2).CuSO_(4)+8NH_(3) to underset("Tetraammine copper (II)(Deep blue solution)")(2[Cu(NH_(3))_(4)]SO_(4)+2OH^(-)+SO_(4)^(2-))`