Solubility

Common Ion Effect

If your coffee already has sugar in it, would it be easy or difficult to dissolve

MORE sugar?

“Common Ion Effect”

Common Ion Effect:

The solubility of a compound is decreased or inhibited if one of its ions is already present in the solvent.

Na+ + I-

Compare the solubility of PbI2 in H20 and in 0.10M NaI solution. Ksp of PbI2 is 8.5 x 10-9

In H2O:

Pb+2 + 2I-PbI2 Ksp = [Pb+2] [I-]2

8.5 x 10-9 = [x][2x]2

8.5 x 10-9 = 4x3

PbI2 = 1.29 x 10-3M

In NaI:

NaI 0.1 0.1 0.1

PbI2 Pb+2 + 2I-

x x 2xKsp = [Pb+2] [I-]2

8.5 x 10-9 = [x][2x + 0.1]2

8.5 x 10-9 = [x][0.1]2

PbI2 = 8.5 x 10-7M

More PbI2 is dissolved in water (1.3 x 10-3M) than in NaI (8.5 x 10-7M)!!!

Consider the equilibrium:

BaCrO4 (s) Ba2+ (aq) + CrO42- (aq)

What would happen if BaCl2 were added? BaCl2 would dissolve Increase the concentration of Ba2+ (common ion) Some Ba2+ will react with CrO4

2- Shifting the equilibrium to the left Forming more BaCrO4

This means that less BaCrO4 has dissolved hence solubility is decreased

Consider the equilibrium:

AgCl (s) Ag+ (aq) + Cl- (aq) If Na2S is added to a solution of AgCl in equilibrium,

Ag2S is precipitated (common ion) this removes Ag+ ions shifting the equilibrium to the right allowing more AgCl to dissolve increasing solubility

If we remove an ion from solution we can shift the equilibrium.

One of the ways of doing this is by forming a less soluble substance