Quick answer: The solubility product Ksp is the equilibrium constant for a sparingly soluble ionic solid dissolving, such as AgCl(s) ⇌ Ag⁺ + Cl⁻, with Ksp = [Ag⁺][Cl⁻]. This AP Chemistry lab on solubility product Ksp gives the aim, the equipment, the safety points, a step-by-step method, sample results and 8 analysis questions; the answers are on a separate page and the lab sheet is a free printable PDF.
How soluble is calcium hydroxide in water? You prepare a saturated solution, titrate it with hydrochloric acid to find [OH⁻], and calculate the molar solubility and Ksp. This is a direct, simple measurement of an equilibrium constant.
Level: AP Chemistry · Curriculum: AP Chem Unit 7
Aim
To find the molar solubility and Ksp of calcium hydroxide at room temperature by titrating its saturated solution with hydrochloric acid.
Equipment
- Solid calcium hydroxide, 2 g, and 250 cm³ of distilled water
- 0.0500 mol/dm³ hydrochloric acid in a burette
- Filter funnel and filter paper, dry beaker, stirring rod
- 25.0 cm³ pipette, filler, conical flask, white tile
- Phenolphthalein indicator, eye protection
Safety
Wear eye protection. Saturated calcium hydroxide (limewater) is an irritant to eyes and skin, and the powder is irritant if inhaled. Rinse splashes with plenty of water. Avoid breathing in the dust when weighing.
Method
- Stir 2 g of Ca(OH)₂ into 250 cm³ of water and leave it for 20 minutes, stirring now and then.
- Filter into a dry beaker, discarding the first 10 cm³.
- Pipette 25.0 cm³ of the filtrate into a conical flask and add 2 drops of phenolphthalein.
- Titrate with the hydrochloric acid to the first permanent loss of pink colour.
- Do a rough titration first, then three more until two results agree within 0.2 cm³.
- Record the room temperature.
Results
Sample results at 24 °C:
| titration | volume of HCl (cm³) | concordant titres (cm³) |
|---|---|---|
| rough | 11.7 | – |
| 1 | 11.3 | 11.3 |
| 2 | 11.2 | 11.2 |
| 3 | 11.4 | 11.4 |
Analysis questions
Question 1 Mean titre
Calculate the mean of the concordant titres.Question 2 Amount of hydroxide
Calculate the amount of OH⁻ in 25.0 cm³ of filtrate (H⁺ + OH⁻ → H₂O).Question 3 Hydroxide concentration
Calculate [OH⁻] in the saturated solution.Question 4 Molar solubility
Calculate [Ca²⁺] and hence the molar solubility of Ca(OH)₂.Question 5 Calculate Ksp
Calculate Ksp for Ca(OH)₂.Question 6 Percentage difference
The accepted Ksp at 25 °C is about 5.0 × 10⁻⁶. Calculate the percentage difference.Question 7 Source of error
Suggest one reason the filtrate could appear less basic than it should, and one improvement.Question 8 Common ion prediction
Predict what happens to the volume of acid needed if you repeat the lab with 0.10 mol/dm³ CaCl₂ in place of water.
