1. Solubility
Common Ion Effect

2. “Common Ion Effect” DIFFICULT
If your coffee already has sugar in it, would it be easy or difficult to dissolve MORE sugar?
“Common Ion Effect”
DIFFICULT

3. Common Ion Effect:
Please write
The solubility of a compound is decreased or inhibited if one of its ions is already present in the solvent.

4. BaCrO4 (s)  Ba2+ (aq) + CrO42- (aq)
Please write
Consider the equilibrium:
BaCrO4 (s)  Ba2+ (aq) + CrO42- (aq)
What would happen if BaCl2 were added?
BaCl2 would dissolve
[Ba2+] would inc. (common ion)
Some Ba2+ will react with CrO42-
equilibrium shifts left
Forming more BaCrO4(s)
This means that less BaCrO4 is dissolved
hence solubility is decreased

5. AgCl (s)  Ag+ (aq) + Cl- (aq)
If we remove an ion from solution we can shift the equilibrium.
One of the ways of doing this is by ppt
Please write
Consider the equilibrium:
AgCl (s)  Ag+ (aq) + Cl- (aq)
If Na2S is added:
Ag+ + S2-  Ag2S (s)
this removes Ag+ ions
shifting the equilibrium to the right
allowing more AgCl to dissolve
increasing solubility

6. In NaI: Na+ + I- NaI ⇄ PbI2 ⇄ Pb+2 + 2I- 0.1 ⇄ 0.1 0.1
Compare the solubility of PbI2 in H20 and in 0.10M NaI solution. Ksp of PbI2 is 8.5 x 10-9
In H2O:
Please write
In NaI:
NaI ⇄
Na+ + I-
PbI2 ⇄
Pb+2 + 2I-
0.1 ⇄
Ksp = [Pb+2] [I-]2
PbI2 ⇄
Pb+2 + 2I-
8.5 x 10-9 = [x][2x]2
x ⇄ x 2x
8.5 x 10-9 = 4x3
Ksp = [Pb+2] [I-]2
PbI2 = 1.29 x 10-3M
8.5 x 10-9 = [x][2x + 0.1]2
8.5 x 10-9 = [x][0.1]2
More PbI2 is dissolved
in water !!!!
PbI2 = 8.5 x 10-7M