To predict whether a precipitate forms after mixing two solutions, calculate the ion concentrations in the combined volume before precipitation. Substitute those concentrations into the ionic product and compare it with the solubility product Ksp.
The 2027 H2 Chemistry 9476 syllabus covers solubility product calculations. This example shows how dilution and the coefficient of an ion in the dissolution equation affect the prediction.
- Write the dissolution equation.
- Calculate the amount of each ion added.
- Divide by the combined volume.
- Use the dissolution coefficients as powers in the ionic product.
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Set up the ionic product
For CaF₂(s) ⇌ Ca²⁺(aq) + 2F⁻(aq), the ionic product is Q = [Ca²⁺][F⁻]². Use the concentrations immediately after mixing, before allowing any solid to form. If Q exceeds Ksp at the stated temperature, the mixture is supersaturated and precipitation is favoured.
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Worked example | Combine 20.0 cm³ and 30.0 cm³
Mix 20.0 cm³ of 0.00200 mol dm⁻³ calcium chloride solution with 30.0 cm³ of 0.00100 mol dm⁻³ sodium fluoride solution. This supplies 4.00 × 10⁻⁵ mol of Ca²⁺ and 3.00 × 10⁻⁵ mol of F⁻. Assume the volumes add and use 0.0500 dm³ for the mixture.
The initial mixed concentrations are 8.00 × 10⁻⁴ mol dm⁻³ Ca²⁺ and 6.00 × 10⁻⁴ mol dm⁻³ F⁻. The ionic product is (8.00 × 10⁻⁴)(6.00 × 10⁻⁴)² = 2.88 × 10⁻¹⁰ mol³ dm⁻⁹. If the question gives Ksp = 4.0 × 10⁻¹¹ mol³ dm⁻⁹, Q is larger and CaF₂ precipitation is predicted.
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Change the volume before mixing
Suppose an extra 50.0 cm³ of water is added to one solution before the solutions are combined. Each initial mixed ion concentration is then halved. Q falls by a factor of eight to 3.60 × 10⁻¹¹ mol³ dm⁻⁹. It is below the stated Ksp, so precipitation is no longer predicted.
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CHAPTER 4 OF 4 · Back to contents
Frequently asked questions
Why is the fluoride concentration squared?
The dissolution equation produces two fluoride ions for each formula unit. The coefficient becomes the power in the expression.
Does Q give the mass of precipitate?
The comparison predicts the direction of change. Finding the mass formed requires an equilibrium calculation or another stated approximation.