Which of the following are the example of double salt?
A. FeSO₄ ·(NH₄)₂ SO₄ · 6 H₂ O
B. CuSO₄· 4 NH₃ · H₂ O
C. K₂ SO₄ · Al₂(SO₄)₃ · 24 H₂ O
D. Fe(CN)₂·4 KCN
Choose the correct answer :

Solution

Chemistry ma’am · 3 steps

Ask what each compound does in water: falls apart into simple ions, or keeps a complex ion intact.

1. The test. A double salt exists only in the solid. In water it breaks up completely into simple ions, so every ion gives its usual test. A complex keeps its complex ion whole in solution.

   Compound	What it is	Ions in water	Type
   A: FeSO₄.(NH₄)₂SO₄.6H₂O	Mohr's salt	Fe²⁺, NH₄⁺, SO₄²⁻	double salt
   B: CuSO₄.4NH₃.H₂O	[Cu(NH₃)₄]SO₄.H₂O	[Cu(NH₃)₄]²⁺, SO₄²⁻	complex
   C: K₂SO₄.Al₂(SO₄)₃.24H₂O	potash alum	K⁺, Al³⁺, SO₄²⁻	double salt
   D: Fe(CN)₂.4KCN	K₄[Fe(CN)₆]	K⁺, [Fe(CN)₆]⁴⁻	complex

2. B and D fail. In B the solution gives no test for free NH₃, because ammonia stays bound to copper. In D it gives no test for free Fe²⁺ or CN⁻, because both stay locked in [Fe(CN)₆]⁴⁻.
3. So. Only A (Mohr's salt) and C (potash alum) are double salts, which is option (d).

The idea to keep. Double salts and complexes can be written the same way, as a dot formula. In water a double salt shows the tests of all its simple ions. A complex keeps its complex ion intact.

Answer from the NTA final answer key. Solution by PrepWiser.