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?
FeSO₄ mixed with (NH₄)₂SO₄ in 1:1 molar ratio forms Mohr's salt, which is a double salt. Double salts dissociate into simple ions completely when dissolved in water. Hence, Fe²⁺ ions are free in solution and give the test for Fe²⁺ ion.
On the other hand, CuSO₄ mixed with aqueous ammonia in 1:4 molar ratio forms a complex or coordination entity. Complex ions do not dissociate into simple ions when dissolved in water. The Cu²⁺ ions are bound within the coordination sphere and are not available as free ions in solution. Therefore, the solution does not give the test for Cu²⁺ ion.
Explanation
This question tests the understanding of the difference between double salts and complexes. The textbook context explicitly states that double salts like Mohr's salt (FeSO₄.(NH₄)₂SO₄.6H₂O) dissociate into simple ions completely when dissolved in water, making Fe²⁺ available for detection. In contrast, complex ions do not dissociate into their constituent metal ions, so Cu²⁺ remains bound in the coordination entity formed with ammonia and cannot be detected by standard tests.
Solution Steps
Step 1: Identify that FeSO₄ + (NH₄)₂SO₄ forms Mohr's salt, a double salt.
Step 2: Apply the concept that double salts dissociate completely into simple ions in water, releasing Fe²⁺.
Step 3: Identify that CuSO₄ + NH₃ forms a complex/coordination compound.
Step 4: Apply the concept that complex ions do not dissociate into simple ions, keeping Cu²⁺ bound.