1. Unit 4 Solutions
solubility constant

2. 4.3.3 The Solubility Product Constant, Ksp
Recall the equilibrium constant expression for any general reaction:
aA + bB cD + dD
Keq= [C]c × [D]d
[A]a × [B]b

3. We can apply this mathematical relationship to solutions
We can apply this mathematical relationship to solutions. We will refer to our equilibrium constant as Ksp, where "sp" stands for "solubility product"
For our silver sulfate saturated solution:
Ag2SO4 (s) Ag+(aq) + SO42-(aq) we can write our solubility product constant expression as

4. Remember that the concentrations of solids and liquids are NOT included in the expression because their concentrations remain constant.
Therefore, we will write our solubility product constant expression for our saturated silver sulfate solution as:
Please note that it is VERY IMPORTANT to include the ion charges when writing this equation

5. Try this example:
Write the expression for the solubility product constant, Ksp, for Ca3(PO4)2.
Step 1: balance equation.
Step 2: Write the expression for Ksp:

6. Solubility Product Tables give Ksp values for various ionic compounds (keep it handy).
Because temperature affects solubility, values are given for specific temperatures (usually 25°C).

7. Ksp provides us with useful information:
A low value of Ksp indicates a substance that has a low solubility (or insoluble); for ionic compounds this means that there will be few ions in solution.
Iron(II) sulfide, FeS, is an example of a low Ksp :
Ksp = 4 ×10-19
In a saturated solution of FeS there would be few Fe2+ or S2- ions.

8. A large value of Ksp indicates a soluble substance; for ionic compounds there will be many ions in solution.
An example lead(II) chloride, PbCl2 Ksp = 1.8 ×10-4
A saturated solution of PbCl2 would have a high concentration of Pb 2+ and Cl - ions.

9. 1. Calculating Ksp when concentration of a saturated solution is known
The concentration of a saturated solution of BaSO4 is × 10-5 M. Calculate Ksp for barium sulfate at 25°C  

10. Example
The concentration of lead ions in a saturated solution of PbI2 at 25°C is 1.3 × 10-3 M. What is its Ksp?  

11. Calculating ion concentrations when Ksp is known.
Ksp for MgCO3 at 25°C is 2.0 × What are the ion concentrations in a saturated solution at this temperature?