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 `(ON_(3))_(2)`

Answer

B

Explanation

In qualitative analysis when `H_(2)S` is passed through an aqueous sloution of salt acidified with dil . HCl a black ppt. of CuS is obtained .
`CuSO_(4)+H_(2)Soverset(dil. HCl)tounderset("blackppt")( 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)to3Cu(NO_(3))_(2)+2NO+3S+4H_(2)O`
`S+2HNO_(3)toH_(2)SO_(4)+NO`
`2Cu^(2+)+SO_(4)^(2-)+2NH_(3)+2H_(2)OtoCu(OH)_(2)*CuSO_(4)+2NH_(4)OH`
`Cu(OH)_(2)*CuSO_(4)+8NH_(3)tounderset("Tetraammine copper"(||)("Deep blue solution"))(2[Cu(NH_(3))_(4)]SO_(4)+2OH^(-)+SO_(4)^(2-)`