Evaluation of K !B
373
When written in this form, to involve only the product of the concentrations
of the ions, the constant K sv is called the "solubility-product constant."
EVALUATION OF K ip
It is found experimentally that, in a saturated solution of AgCl at room temperature, the concentration of dissolved salt is 0.00191 g/liter, or 1.33 x 10~
5 mole/
liter. When a molecule of AgCl dissolves, it dissociates to give an Ag
+ ion and a
Cl~ ion. Therefore, the concentrations of Ag
+ and Cl~ in the saturated solution
are each 1.33 x 10~
5 M. Substituting these concentrations, we have
[Ag+][Cl-] = (1.33 x 105
)
2 = 1.78 x lO"
10 = K sv
When the precipitate has different numbers of positive and negative ions, the
calculation of K sp is a little more complicated, as illustrated by the next problem.
PROBLEM:
The solubility of Mg(OH) 2 is 8.34 x 10"
4 g/100 ml at 25.0°C. Calculate the K sp
value.
SOLUTION:
The chemical equilibrium involved is
Mg(OH) 2(s) <± Mg
2+ + 20HEach mole of Mg(OH) 2 that dissolves produces 1 mole of Mg
2+ and 2 moles
of OH-.
= 1.43 x 10~
4 M
[Mg
2+ ] = 1.43 x 10-" M
[OH-] = 2.86 x 104 M
Based on the chemical equation, the K it , expression is
KSP = [Mg
2+ ][OH-]
2
= (1.43 x 104
)(2.86 x l(r
4 )
2
= 1.16 x 10-"
Table 24-1 lists experimentally-determined values of K^ v for some common
insoluble compounds.