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

By mixing FeSO₄ solution with (NH₄)₂SO₄ solution in 1 : 1 molar ratio, a double salt is obtained which is called Mohr's salt (FeSO₄.(NH₄)₂SO₄.6H₂O).

It ionizes in the following manner:

\[\ce{FeSO4.(NH4)2SO4.6H4O -> F^{2+} + 2NH^+_4 + 3SO^{2-}_4 + 6H2O}\]

It gives the test of Fe²⁺ ions due to the presence of Fe²⁺ ions in the solution. When CuSO₄ solution is mixed with aqueous ammonia in 1 : 4 molar ratio, hybrid salt [Cu(NH₄)₄]SO₄ is obtained. It ionizes in the solution as follows:

\[\ce{[Cu(NH3)4]SO4 -> [Cu(NH3)4]^{2+} + SO^{2-}_4}\]

The hybrid ion [Cu(NH₃)₄]²⁺ does not re-ionise to form Cu²⁺ ions. Hence the solution does not give the test for Cu²⁺ ions.