FeSO4 solution mixed with (NH4)2SO4 solution in 1:1 molar ratio gives the test of Fe2+ ion but CuSO4 solution mixed with aqueous ammonia in 1:4 molar

The reaction is given below:

FeSO4+(NH4)2SO4+6H2O ---> FeSO4(NH4)2SO4.6H2O (Mohr Salt)

FeSO4, when reacted with (NH4)SO4, does not form any complex whereas they form a double salt, FeSO4.(NH4)2SO4.6H2O -  (Mohr salt) which dissociates into ions in the solution. So, it gives the test of Fe2+ ions.

CuSO4 +4NH3 +5H2O---->[Cu(NH3)4SO4].5H2O

 CuSO4 solution when mixed with aqueous ammonia in 1: 4 molar ratio forms a complex with formula [Cu(NH3)]SO4 in which the complex ion, [Cu(NH3)4]2+ does not dissociate to give Cu2+ ions. Therefore, it does not give the tests of the Cu2+ ion.