1. Chapter 9 Chemical Equilibrium
9.6 Equilibrium in Saturated Solutions

2. Saturated Solution A saturated solution
contains the maximum amount of dissolved solute
contains solid solute
is an equilibrium system:
rate of dissolving = rate of recrystallization
solid ions in solution

3. Solubility Product Constant
The solubility product constant for a saturated solution
gives the ion concentrations at constant temperature
is expressed as Ksp
does not include the solid, which is constant
Fe(OH)2(s) Fe2+(aq) + 2OH−(aq)
Ksp = [Fe2+] [OH−]2

4. Learning Check Write the Ksp expression for each of the following:
A. FeS(s)
B. Ag2CO3(s)
C. Ca(IO3)2(s)

5. Solution Write the Ksp expression for each of the following:
A. FeS(s) Fe2+(aq) + S2−(aq)
Ksp = [Fe2+][S2−]
B. Ag2CO3(s) 2Ag+(aq) + CO32− (aq)
Ksp = [Ag+]2 [CO32−]
C. Ca(IO3)2(s) Ca2+(aq) + 2IO3−(aq)
Ksp = [Ca2+][IO3−]2

6. Guide to Calculating Ksp

7. Example of Calculating Solubility Product Constant
Calculate the Ksp of PbSO4 (solubility 1.4 x 10–4 M).
STEP 1 Write the equilibrium equation for dissociation:
PbSO4(s) Pb2+(aq) + SO42−(aq)
STEP 2 Write the Ksp expression:
Ksp = [Pb2+][SO42−]
STEP 3 Substitue molarity values and calculate:
Ksp = (1.4 x 10–4) x (1.4 x 10–4)
= 2.0 x 10–8

8. Examples of Solubility Product Constants

9. Learning Check What is the Ksp value of PbF2 if the solubility at
25 C is 2.6 x 10–3 M?
1) 1.4 x 10–5
2) 6.8 x 10–6
3) 7.0 x 10–8

10. Solution
3) 7.0 x 10-8
STEP 1 Write the equilibrium equation for dissociation:
PbF2(s) Pb2+(aq) + 2F−(aq)
STEP 2 Write the Ksp expression:
Ksp = [Pb2+][F−]2
STEP 3 Substitute molarity values and calculate:
[Pb2+] = 2.6 x 10–3 M
[F−] = 2 x [Pb2+] = 5.2 x 10–3 M
Ksp = (2.6 x 10–3) x (5.2 x 10–3)2 = 7.0 x 10–8

11. Molar Solubility (S) The molar solubility (S) is
the number of moles of solute that dissolve in 1 L of solution
determined from the formula of the salt
calculated from the Ksp

12. Calculating Molar Solubility (S)

13. Solubility Calculation
Determine the solubility (S)2 of SrCO3 (Ksp = 5.4 x 10–10).
STEP 1 Write the equilibrium equation for dissociation:
SrCO3(s) Sr2+(aq) + CO32−(aq)
STEP 2 Write the Ksp expression:
Ksp = [Sr2+][CO32−]
STEP 3 Substitute S for the molarity of each ion into Ksp:
Ksp = [Sr2+][CO32−] = [S][S]
= S2 = 5.4 x 10−10
STEP 4 Calculate the solubility, S:
S = = x 10−5 M

14. Learning Check
Calculate the solubility (S) of PbSO4 , if the Ksp = 1.6 x 10–8.
1) 1.3 x 10–4 M
2) 4.0 x 10–4 M
3) 2.6 x 10–16 M

15. Solution
1) 1.3 x 10–4 M
STEP 1 Write the equilibrium equation for dissociation:
PbSO4(s) Pb2+(aq) + SO42−(aq)
STEP 2 Write the Ksp expression:
Ksp = [Pb2+][SO42−]
STEP 3 Substitute S for the molarity of each ion into Ksp:
Ksp = [Pb2+][SO42−] = [S][S]
= S2 = 1.6 x 10–8
STEP 4 Calculate the solubility, S:
S = = 1.3 x 10–4 M