Solubility Product Calculations (Cambridge (CIE) A Level Chemistry): Revision Note
Exam code: 9701
Solubility Product Calculations
Calculations involving the solubility product (Ksp) may include::
Calculating the solubility product of a compound from its solubility
Calculating the solubility of a compound from the solubility product
Worked Example
Calculating the solubility product of a compound from its solubility
Calculate the solubility product of a saturated solution of lead(II) bromide, PbBr2, with a solubility of 1.39 x 10-3 mol dm-3.
Answer
Step 1: Write down the equilibrium equation:
PbBr2 (s) ⇌ Pb2+ (aq) + 2Br- (aq)
Step 2: Write down the equilibrium expression:
Ksp = [Pb2+(aq)] [Br- (aq)]2
Step 3: Calculate the ion concentrations in the solution:
[PbBr2(s)] = 1.39 x 10-3 mol dm-3
The ratio of PbBr2 to Pb2+ is 1:1
[Pb2+(aq)] = [PbBr2(s)] = 1.39 x 10-3 mol dm-3
The ratio of PbBr2 to Br- is 1:2
[Br-(aq)] = 2 x [PbBr2(s)] = 2 x 1.39 x 10-3 mol dm-3 = 2.78 x 10-3 mol dm-3
Step 4: Substitute the values into the expression to find the solubility product:
Ksp = (1.39 x 10-3) x (2.78 x 10-3)2
Ksp = 1.07 x 10-8
Step 6: Determine the correct units of Ksp:
Ksp = (mol dm-3) x (mol dm-3)2
Ksp = mol3 dm-9
Therefore, the solubility product is 1.07 x 10-8 mol3 dm-9
Worked Example
Calculating the solubility of a compound from its solubility product
Calculate the solubility of a saturated solution of copper(II) oxide, CuO, with a solubility product of 5.9 x 10-36 mol2 dm-6.
Answer
Step 1: Write down the equilibrium equation:
CuO (s) ⇌ Cu2+ (aq) + O2- (aq)
Step 2: Write down the equilibrium expression:
Ksp = [Cu2+ (aq)] [O2- (aq)]
Step 3: Simplify the equilibrium expression:
The ratio of Cu2+ to O2- is 1:1
[Cu2+(aq)] = [O2-(aq)] so the expression can be simplified to:
Ksp = [Cu2+ (aq)]2
Step 4: Substitute the value of Ksp into the expression to find the concentration:
5.9 x 10-36 = [Cu2+ (aq)]2
[Cu2+ (aq)] =
[Cu2+ (aq)] = 2.4 x 10-18 mol dm-3
Since [CuO (s)] = [Cu2+ (aq)], the solubility of copper oxide is 2.4 x 10-18 mol dm-3
Examiner Tips and Tricks
Remember that the solubility product is only applicable to very slightly soluble salts and cannot be used for soluble salts such as:
Group 1 element salts
All nitrate salts
All ammonium salts
Many sulfate salts
Many halide salts (except for lead(II) halides and silver halides)
Unlock more revision notes. It’s free!
By signing up you agree to our Terms and Privacy Policy.
Already have an account? Log in
Did this page help you?