Many ionic compounds are sparingly soluble – a good example is barium sulphate which has a solubility of 1.05 x 10-5 mol dm-3. This means that if we weighed out 1.05 x 10-5 mol (0.00245g) and dissolved it in 1000cm3 of water, we would have a saturated solution in which solid, undissolved barium sulphate would be in an equilibrium with barium and sulphate ions.
BaSO4(s) ⇌ Ba2+(aq) + SO42-(aq)
The equilibrium constant for this system is called the solubility product, Ksp.
There are so few barium and sulphate ions in solution that we can assume they are electrically neutral (wrapped up in their hydration shells). This assumption does not hold true for more soluble salts (with an ion concentration of more than 0.01 mol dm-3) where the ions in solution do interact with each other electrically so their effective concentration is not the same as their actual concentration.
We can calculate Ksp (and its units) in the usual way:
Ksp = (1.05 x 10-5)2 = 1.10 x 10-10 mol2 dm-6
So what does Ksp tell us?
If we were to mix two solutions, one containing Ba2+ ions and one containing SO42- ions, and the product of the ion concentration was less than Ksp, then we would know that equilibrium has not been established as no solid barium sulphate would be present, the solution would not be saturated. On the other hand, if Ksp was exceeded by the product of the ion concentration, then the solution would be saturated and we would observe barium sulphate precipitating out.
Calculating Ksp
Calculate the solubility product of chromium(III) oxide given that it has a solubility of 1.39 x 10-8 mol dm-3.
Cr(OH)3(s) ⇌ Cr3+(aq) + 3OH–(aq)
Calculating the solubility of a sparingly soluble salt
Calculate the solubility of iron(II) hydroxide given that its solubility product is 6.0 x 10-15 mol3 dm-9.
Practice questions
- (a) Write an expression for the solubility product of the following sparingly soluble salts:
| Salt | Solubility / mol dm-3 |
| CdCO3 | 1.58 x 10 -7 |
| CaF2 | 2.15 x 10-4 |
(b) Calculate a value for Ksp for each of the salts and determine the correct units.
- Calculate the solubility of each of the following sparingly soluble salts:
| Salt | Solubility product |
| CuS | 6.30 x 10-36 mol2 dm-6 |
| Ca(OH)2 | 5.5 x 10-6 mol3 dm-9 |
Answers
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