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 ratio does not give the test of Cu2+ ion. Explain why?

Answer ::

FeSO4 does not form any complex with (NH4)2SO4. Instead, it forms a double salt FeSO4. (NH4)2SO4.6H2O which dissociates completely into ions. CuSO4 when mixed with NH3 forms a complex [CU(NH3)4]SO4 in which the complex ion [CU(NH3)4]2+ does not dissociate to give Cu2+ ion.