Untitled - Kelly Walsh High School

Buffers and Other Equilibria 239
Don’t Forget!
There is never a denominator in a K sp expression.
For this particular salt, the value of the K sp is 1.6 10 8 at 25C. If we know the
value of the solubility product constant, then we can determine the concentration
of the ions. In addition, if we know one of the ion concentrations, then we
can find the K sp.
The K sp of magnesium fluoride in water is 8 10 8 . How many grams of magnesium
fluoride will dissolve in 0.250 L of water?
MgF2(s) K Mg2 (aq) 2 F (aq)
Ksp [Mg2 ][F ] 2 8 108 For every mole of MgF 2 that dissolves, 1 mol of Mg 2+ and 2 mol of F form:
K sp (x)(2x) 2 4 x 3 8 10 8
x 3 10 3 [Mg 2 ]
a 0.05 g MgF2 3 103 mol Mg2 b (0.250 L) a
L
1 mol MgF2 1 mol Mg2b a62.3 g MgF2 b
1 mol MgF2 When solving common-ion-effect problems, calculations like the ones above
involving finding concentrations and Ksp’s can still be done, but the concentration
of the additional common ion will have to be inserted into the solubility
product constant expression. Sometimes, if the Ksp is very small and the common
ion concentration is large, we can simply approximate the concentration of
the common ion by the concentration of the ion added.
Calculate the silver ion concentration in each of the following solutions:
a. Ag2CrO4(s) water
b. Ag2CrO4(s) 1.00 M Na2CrO4 Ksp 1.9 1012 a. Ag2CrO4(s) K 2 Ag 2 (aq) CrO4 (aq)
2 x x
Ksp 1.9 1012 (2 x) 2 (x) 4 x3 x 7.8 105 [Ag ] 2 x 1.6 104 M