In potash alum, the ratio of K+ and SO₄²⁻ ions is:
- A.1:2✓
- B.2:1
- C.2:3
- D.3:2
Correct Answer
(A) 1:2
Solution & Explanation
Concept — Potash alum is a double salt with the formula K₂SO₄·Al₂(SO₄)₃·24H₂O. Step 1 — Count the ions in one formula unit. K⁺: the K₂SO₄ part supplies 2 K⁺ ions. SO₄²⁻: K₂SO₄ gives 1 sulphate and Al₂(SO₄)₃ gives 3 sulphates, so total = 1 + 3 = 4 SO₄²⁻ ions. Step 2 — Form the ratio. K⁺ : SO₄²⁻ = 2 : 4 = 1 : 2. (Quick check via charges: 2 K⁺ = +2, 2 Al³⁺ = +6, total cation charge +8, balanced by 4 SO₄²⁻ = −8. Consistent.) Hence the ratio of K⁺ to SO₄²⁻ ions is 1 : 2 — option A.
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