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?

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.