KUAT Chemistry Sample Question: How to Solve a Ksp Question Step by Step

Ksp questions become much easier when you stop trying to remember a formula for every salt.

Start with the dissolution equation.

The coefficients tell you the ion concentrations. The ion concentrations then give you the Ksp expression.

Try the question

Calcium fluoride dissolves according to:

CaF₂(s) ⇌ Ca²⁺(aq) + 2F⁻(aq)

If the molar solubility of CaF₂ is s, which expression equals Ksp?

A) s²
B) 2s²
C) 4s³
D) 8s³

Step 1 — Translate molar solubility into ion concentrations

The equation tells us:

1 CaF₂ → 1 Ca²⁺ + 2 F⁻

If s moles per litre of CaF₂ dissolve, then:

[Ca²⁺] = s

and:

[F⁻] = 2s

That small coefficient 2 is the key to the whole question.

Step 2 — Write the Ksp expression

The solid is not included in the Ksp expression.

Ksp = [Ca²⁺][F⁻]²

The fluoride concentration is squared because its coefficient in the balanced equation is 2.

Step 3 — Substitute the concentrations

Ksp = (s)(2s)²

(2s)² = 4s²

Therefore:

Ksp = s × 4s² = 4s³

The correct answer is C) 4s³.

Why the wrong answers are tempting

A) s² ignores the 2 in front of fluoride and does not build the expression correctly.

B) 2s² notices the coefficient but fails to square the full fluoride concentration.

D) 8s³ gives too much power to the coefficient. (2s)² = 4s², not 8s².

The safer method

Do not memorise “CaF₂ means 4s³.” Memorise the process:

1. write the balanced dissolution equation;
2. convert coefficients into equilibrium concentrations;
3. write the Ksp expression;
4. substitute;
5. simplify.

Then you can handle a different salt even if the final expression is completely different. The rest of the KUAT Chemistry topics reward the same habit.

One last check

Suppose the salt were:

MX₂(s) ⇌ M²⁺(aq) + 2X⁻(aq)

If its molar solubility were s, the same structure would give [M²⁺] = s, [X⁻] = 2s, and therefore Ksp = 4s³.

The letters changed. The reasoning did not.

For more worked questions by subject, start from the KUAT sample-test guide.