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

Explanation

when `FeSO_(4) and (NH_(4))_(2)SO^(4)` soltuions are mixed in 1 : 1 molar ratio, a double salt known as Mohrs salt is formed. It has the formula `FeSO_(4). (NH_(4))_(2)SO_(4). 6H^(2)O`. In aqueous solution. The salt dissociates as :
`FeSO_(4). (NH_(4))_(2)SO_(4). 6H^(2)O (aq) ((aq)/rarr) Fe^(2) (aq) + 2NH_(4)^(+) (aq) + 2SO_(4)^(2-) (aq) + 6H_(2)O`
the solution gives the tests for all the icons including `FE^(2+)` ions. on the other hand, when `CuSO_(4) and NH_(3)` are mixed in the molar ratio of 1 : 4 in solution, a comples `[Cu(NH_(3))_(4)] SO_(4)` is formed. since the `Cu^(2+)` ions are a part of the complex entity (enclosed in square bracked), It will not give their characteristic tests as are given by `Fe^(2+)` ions.