1. Solubility Product Constant (Ksp)

2. The Terms
Solubility: - a measure of the ability of a substance to dissolve Saturated Solution: - has the maximum amount of solute dissolved in a given volume of solvent at a given temperature Unsaturated: - has less than the maximum… Supersaturated: - has more than the maximum…

3. The Terms
Solubility: - measured in g of solute per 100 mL of solution units: (g/100 mL) Molar Solubility: - measured in mole of solute per litre of solution units: (mol/L)

4. Solubility Conversions?
e.g. MgF2
solubility = 1.72 x 10-3 g/100 mL
molar solubility = 2.76 x 10-4 mol/L

5. Solubility Product Constant
For saturated solutions at equilibrium we use the solubility product constant symbolized Ksp
Ex:
Zn(OH)2(s) ↔ Zn2+(aq) + 2 OH1-(aq)
Since Ksp is always calculated by just multiplying concentrations, it is called a “solubility product” constant - Ksp.

6. Solubility Product Constant
Ex:
Zn(OH)2(s) ↔ Zn2+(aq) + 2 OH1-(aq)
Note: Notice that Zn(OH)2(s) is left out of the expression! Why?
Physical states matter when you are writing the eqm law equation!
See Tables for values of Ksp

7. Solubility Product Constant (Ksp)
Note-
Used for saturated solutions.
Used for ionic compounds of low solubility, because under ordinary conditions highly soluble ionic compounds do not form precipitates
Units are omitted from values.
The lower the value of Ksp the lower the solubility of the compound.

8. Sample Problems
These problems are similar to K except used for solubility.
We seldom require an “ICE” table for Ksp problems.

9. Sample Problems (Ans: 8.8 x 10-12 )
Zinc carbonate is a hard, slightly soluble salt Calculate the solubility (in g/100 mL) given the molar solubility of ZnCO3 is 3.87 x 10-6 mol/L.
(Ans: 4.85 x 10-5 g/100 mL)
2. A chemist finds that the molar solubility of silver carbonate, Ag2CO3, is 1.3 x 10-4 mol/L at 25C Calculate Ksp for silver carbonate.
(Ans: 8.8 x )

10. Will a Precipitate Form?
Use trial ion product, Qsp, and compare the value to Ksp.
If Qsp < Ksp
rxn is not at eqm
soln is unsaturated, ppt is not present
If Qsp > Ksp
soln is supersaturated, ppt is present
If Qsp = Ksp
rxn is at eqm
soln is saturated, ppt is not present

11. Sample Problems
3. Consider an aqueous solution containing Ag1+ and Cl1- ions, each at a concentration of x 10-5 mol/L. Will precipitation occur?
(ans: Qsp < Ksp , no ppt forms)
4. A chemist mixes mL of 0.25 mol/L Pb(NO3) 2 with mL of mol/L NaBr. a) Write the balanced chemical equation for the reaction b) Does a precipitate form? Hint: Recall dilution.
(ans: Qsp > Ksp , ppt forms)

12. Ions in Aqueous Solutions
So far we have studied the solubility of ionic compounds (salts) in pure water.
Equilibrium will shift if
- Another compound containing a common ion is added or
- Another compound that removes an ion already in solution is added

13. Adding a Common Ion
The solubility of MgF2 in pure water is 2.6 x 10-4 mol/L. What happens to the solubility if we dissolve the MgF2 in a solution of NaF, instead of pure water? Ans: Solubility is lowered due to the addition of the fluoride ion (F1-)
common ion

14. Explaining the Common Ion Effect
The presence of a common ion in a solution will lower the solubility of a salt.
Le Chatelier’s Principle:
The addition of the common ion will shift the solubility equilibrium backwards (left). This means that there is more solid salt in the solution and therefore the solubility is lower!
Ex: MgF2(s) ↔ Mg 2+(aq) F1-(aq)

15. Example
1. The solubility of lead (II) chromate (PbCrO4(s)) in water is 4.8 x 10-7 mol/L. Determine the solubility of PbCrO4(s) in a 0.10 mol/L solution of Na2CrO4. For lead chromate, Ksp = 2.3 x (Ans: 2.3 x mol/L) 2. Try p492 #10