1. CH 17: Solubility and Complex-Ion Equilibria
Renee Y. Becker
CHM 1046
Valencia Community College

2. Solubility Equilibria
Solubility Product Constant, Ksp
Same as Kc, Kp, Kw, Ka, & Kb Prod / reactant Coefficients are exponents, omit solids and pure liquids
CaF2(s)  Ca2+(aq) F-(aq)
Ksp = [Ca2+][F-]2

3. Write Ksp expressions for the following a) Mg(OH)2 SrCO3 Ca3(AsO4)2
Fe(OH)3

4. example
The solubility of silver bromate, AgBrO3, in water is g/L. Calculate Ksp

5. example
Calculate Ksp for copper(II)iodate, Cu(IO3)2. The solubillity of copper(II)iodate in water is 0.13 g/100mL

6. Precipitation of Ionic Compounds
Ion Product (IP)
Same as Ksp but at some time, t, snapshot like Qc, reaction quotient
CaF2(s)  Ca F-
IP = [Ca2+][F-]2
If IP > Ksp solution is supersaturated and precipitation will occur
If IP = Ksp the solution is saturated and equilibrium exists
If IP< Ksp the solution is unsaturated and ppt will not occur

7. Example
Will a precipitate form on mixing equal volumes of the following solutions?
3.0 x 10-3 M BaCl2 and 2.0 x 10-3 M Na2CO3 (Ksp = 2.6 x 10-9 for BaCO3)
1.0 x 10-5 M Ba(NO3)2 and 4.0 x 10-5 M Na2CO3

8. From each of the following ion concentrations in a solution, predict whether a ppt will form in the solution
A) [Ba2+] = M [F-] = M
B) [Pb2+] = M [Cl-] = 0.15 M

9. The following solutions are mixed: 1 L of a 0.00010 M NaOH
example
The following solutions are mixed:
1 L of a M NaOH
1 L of a M MgSO4
Is a ppt expected, explain

10. Measuring Ksp and Calculating Solubility from Ksp
Example: A saturated solution of Ca3(PO4)2 has [Ca2+] = 2.01 x 10-8 M and
[PO43-] = 1.6 x 10-5 M.
Calculate Ksp for Ca3(PO4)2

11. Factors that Affect Solubility
1. The Common ion effect
MgF2(s)  Mg2+(aq) F-(aq)
If we try dissolve this in a aqueous solution of NaF the equilibrium will shift to the left. This will make MgF2 less soluble
2. Formation of Complex ions
Complex ion: An ion that contains a metal cation bonded to one or more small molecules or ions, NH3, CN- or OH-
AgCl(s)  Ag+ + Cl-
Ag NH3  Ag(NH3)2+
Ammonia shifts the equilibrium to the right by tying up Ag+ ion in the form of a complex ion

12. Factors that Affect Solubility
3. The pH of the solution
a) An ionic compound that contains a basic anion becomes more soluble as the acidity of the solution increases
CaCO3(s)  Ca CO32-
H3O+ + CO32-  HCO H2O
Net: CaCO3(s) + H3O+  Ca HCO3- + H2O
Solubility of calcium carbonate increases as the pH decreases because the CO32- ions combine with protons to give HCO3- ions. As CO32- ions are removed from the solution the equilibrium shifts to the right to replenish the carbonate
PH has no effect on the solubility of salts that contain anions of strong acids because these anions are not protonated by H3O+