Write solubility-product expressions
Define Ksp and write its equilibrium expression from the balanced dissolution equation of a sparingly soluble salt.
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 25. Use the equilibrium expressions and data stated in the question; written practice is self-checked and human subject-teacher review remains pending.
Before you begin
- Use AS equilibrium expressions, mole calculations, logarithms, and Brønsted–Lowry acid–base ideas.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Why is a pure solid omitted from a Ksp expression?
What you will learn
- Define Ksp for a saturated solution
- Use stoichiometric coefficients as powers
- Omit the pure solid from the equilibrium expression
Ksp describes an ionic solid at equilibrium
For a sparingly soluble solid AₘBₙ(s) ⇌ mAⁿ⁺(aq) + nBᵐ⁻(aq), Ksp = [Aⁿ⁺]ᵐ[Bᵐ⁻]ⁿ in a saturated solution at a stated temperature.
The solid does not appear in Ksp because its activity is constant. Each dissolved ion concentration is raised to the coefficient in the balanced dissolution equation.
Write and check the expression from the equation
For AgCl(s) ⇌ Ag⁺ + Cl⁻, Ksp = [Ag⁺][Cl⁻]. For PbI₂(s) ⇌ Pb²⁺ + 2I⁻, Ksp = [Pb²⁺][I⁻]².
Do not include the solid or alter the formula to match the number of atoms in a formula unit without first balancing the dissolution equation. The numerical units of Ksp depend on the convention used; use the supplied data consistently.
Pause and explain
What is Ksp for PbI₂(s) ⇌ Pb²⁺ + 2I⁻?
Worked example
Write the Ksp expression for CaF₂(s) ⇌ Ca²⁺(aq) + 2F⁻(aq).
Show the worked solution
- Read the coefficient of each dissolved ion.
- Use each coefficient as a concentration power.
- Omit CaF₂(s).
Answer Ksp = [Ca²⁺][F⁻]².
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Write Ksp for Mg(OH)₂(s) ⇌ Mg²⁺(aq) + 2OH⁻(aq).
Give me a hint
Use the ion coefficients as exponents and omit the solid.
Compare my reasoning
- Mg²⁺ has coefficient 1.
- OH⁻ has coefficient 2.
- Multiply the concentration terms.
Ksp = [Mg²⁺][OH⁻]².
Look for these in your work
- Both ions included
- Exponent matches equation
- Solid omitted
A learner writes Ksp = [AgCl][Ag⁺][Cl⁻] for AgCl(s). Correct the expression.
Give me a hint
The solid is pure; focus on aqueous species.
Compare my reasoning
- Remove AgCl(s) from the expression.
- Keep the two aqueous ions.
- Use coefficient 1 for each.
Ksp = [Ag⁺][Cl⁻]. The pure solid AgCl is not included.
Look for these in your work
- Solid removed
- Aqueous ions retained
- Stoichiometry respected
Write the dissolution equation and Ksp expression for Al₂O₃(s) ⇌ 2Al³⁺ + 3O²⁻ as a formal stoichiometric exercise.
Give me a hint
Use the supplied equation exactly; this question tests expression construction.
Compare my reasoning
- Read coefficients 2 and 3.
- Raise [Al³⁺] to power 2 and [O²⁻] to power 3.
- Omit the solid.
For the supplied formal equation, Ksp = [Al³⁺]²[O²⁻]³.
Look for these in your work
- Supplied coefficients used
- Powers correctly applied
- Solid omitted
Common mistakes
- Including the pure solid in Ksp
- Forgetting to square the ion with coefficient 2
- Writing an expression before balancing dissolution
- Mixing concentrations from different temperatures
What can you explain now?
Why is a pure solid omitted from a Ksp expression?
Compare with the explanation
Its activity is constant
The solid's amount does not enter the equilibrium expression.
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Make it stick
How does a balanced dissolution equation determine the powers and species in Ksp?
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