How many cations will be produced from a solution of potassium aluminium tetraoxosulphate (VI)?

Analyze the question: Potassium aluminium tetraoxosulphate(VI) is alum, KAl(SO₄)₂·12H₂O. We need to count the different cations produced when it dissolves in water.

Concept: Alum is a double salt. It contains two different metal cations and one type of anion. When dissolved, it dissociates completely into the ions of both original salts (K₂SO₄ and Al₂(SO₄)₃).

Solution:

KAl(SO₄)₂ → K⁺ + Al³⁺ + 2SO₄²⁻

• Cations: K⁺ and Al³⁺, which makes 2 types of cation
• Anion: SO₄²⁻ (only 1 type)

So the solution contains 2 cations (K⁺ and Al³⁺).

Checking the options:

• A (3) and B (4) are too many. They might count all the ions, but there are only 3 types of ion in total, and one of them is an anion.
• C (1) ignores the fact that alum is a double salt.
• D (2) matches K⁺ and Al³⁺.

Tip: A double salt gives the ions of both parent salts in solution. For alum, remember "two different cations, one anion": K⁺, Al³⁺ and SO₄²⁻. Read carefully whether the question asks for cations only or for all ions. Counting all ions (3 types) would lead you to the wrong option.