Ncert Exercises & Solutions

9.2 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?

NEETprep Answer:

Step 1:

FeSO₄ solution mixed with (NH₄)₂SO₄ solution in 1:1 molar ratio then double salt is formed. The reaction is as follows:

(NH₄)SO₄ + FeSO₄ + 6H₂O → (NH₄)₂Fe(SO₄)₂·6H₂O (Mohr's salt)

A double salt is an addition compound that is stable in the solid state but breaks up into its constituent ions in the dissolved state. These compounds exhibit individual properties of their constituents. For e.g. (NH₄)₂Fe(SO₄)₂·6H₂O breaks into Fe²⁺, NH₄⁺, and SO₄^2- ions. Hence, it gives a positive test for Fe²⁺ ions.

Step 2:

CuSO₄ solution mixed with aqueous ammonia in 1:4 molar ratio then complex or coordination compound is formed.

The reaction is as follows:
CuSO₄ + 4NH₃ + 5H₂O →[Cu(NH₃)₄]SO₄.5H₂O (tetraamminocopper(ii)sulphate)

A coordination compound is an addition compound that retains its identity in the solid as well as in the dissolved state. The non-ionizable part of a complex compound consists of a central transition metal ion surrounded by neighboring atoms or groups enclosed in a square bracket. Hence, complex does not dissociate into its constitutent and doesnot give Cu²⁺ ion test.
However, the individual properties of the constituents are lost

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