1. Solubility Constant

2. Solubility Some ionic compounds dissolve in water while other will not
It is not always all or nothing
Others are slightly soluble
A fraction of the ionic compound will dissolve in water while some
A fraction of the compound will dissociate into ions

3. Equilibrium
The solution of a slightly soluble solid exist in equilibrium
Ions are separating and reforming at constant rate
Every time a molecule dissolves a new one forms from ions reforming
The expression can be written as
AB2 ⇄ A + 2B
The reaction goes forward and backward at the same rate

4. Solubility Constant
At equilibrium, the concentrations of the free ions are known
Mathematical relationship exists between the free ions
For slightly soluble solids the relationship exits
AB2  A+ + 2B-
Ksp = [A+] x [B-]2
Note the coefficient becomes the exponent
The value for Ksp is known for most subtances

5. Value of Ksp There is a direct relationship between Ksp and solubility
Ksp > 1 indicates a highly soluble substance
Ksp < 1 indicates a slightly soluble substance
The smaller the number the lower the solubility

6. Example
What is the concentration of Ca+2 ions when CaCO3 is placed in water if Ksp = 3.3 x 10-9?
3.3 x 10-9 = [Ca+2] x [CO3-2]
Since the concentrations of the ions are equal
The concentrations can written as X
3.3 x 10-9 = X ∙ X
3.3 x 10-9 = X2
find the square root of 3.3 x 10-9
X = 5.7 x 10-5