`FeSO_4`, solution mixed with `(NH_4)_2SO_4`, solution in 1:1 molar ratio gives the test for `Fe^(2+)` ion but `CuSO_4`, solution mixed with aqueous ammonia in1:4 molar ratio does not give the test for `Cu^(2+)` ion. Explain.

Explanation

When `FeSO_(4)and(NH_(4))_(2)SO_(4)` solution are mixed in `1:1` molar ratio, Mohr.s salt (a double salt) is formed.
`FeSO_(4)("aq")+(NH_(4))_(2)SO_(4)("aq")rarrFeSO_(4)*(NH_(4))_(2)SO_(4)*6H_(2)O`
`FeSO_(4)*(NH_(4))_(2)SO_(4)*6H_(2)OhArrFe^(2+)("aq")+2NH_(4)^(+)("aq")+2SO_(4)^(2-)("aq")+6H_(2)O`
Because `Fe^(2+)` ions are formed on dissolution og Mohr.s salt, its aqueous solution gives the test of `Fe^(2+)` ions. But when `CuSO_(4)` is mixed with ammonia, following reaction occurs.
`CuSO_(4)("aq")+4NH_(3)("aq")rarr[Cu(NH__(3))_(4)]SO_(4)`
This complex does not produce `Cu^(2+)` ion, so the solution of `CuSO_(4)andNH_(3)` does not give the test of `Cu^(2+)` ion.