Question

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 1

i. 1 : 1 molar mixture of FeSO₄ and (NH₄)₂SO₄ forms a double salt, FeSO₄.(NH₄)₂SO_4.6H₂O (Mohr’s salt) which exists only in solid state. In aqueous solution, it dissociates into ions Fe2+, NH^{4 +} and SO₄^2-. Hence, it gives the test of Fe²⁺ ion.

ii. 1 : 4 molar mixture of CuSO₄ and ammonia forms a coordination compound, [Cu(NH₃)₄]SO₄. In aqueous solution, it retains its identity and does not dissociate into ions. Hence, it does not give the
test of Cu²⁺ ion.