Acid-Base Equilibria and Solubility Equilibria

1
16.6
2
Calculating Ksp from Solubility
1.3 x 10-4 moles of Mg(OH)2 solid dissolves in
1.00 L of water. What is the Ksp of Mg(OH)2 ?
0.00
0.00
Ksp MgOH- 2
s
2s
Ksp (s)(2s)2
s
2s
Ksp 4s3
s 1.3 x 10-4 moles
1.3 x 10-4 M
1.00 L
Ksp
4 (1.3 x 10-4 M Mg(OH)2) 3
8.9 x 10-12
3
If 2.00 mL of 0.200 M NaOH are added to 1.00 L of
0.100 M CaCl2, will a precipitate form?
The ions present in solution are Na, OH-, Ca2,
Cl-.
Only possible precipitate is Ca(OH)2 (solubility
rules).
Is Qsp gt Ksp for Ca(OH)2?
Total Volume 1.00 L 0.002 L 1.00 L
Ca20 0.100 M
OH-0 (0.002 L)(0.200 M) 4.0 x 10-4 M
0.100 x (4.0 x 10-4)2 1.6 x 10-8
from table
Ksp Ca2OH-2 8.0 x 10-6
Qsp lt Ksp
No precipitate will form
16.6
4
The Common Ion Effect and Solubility
Ksp
Multiple sources of ions still have to obey
solubility limit defined by solubility constant
Ksp ion
Remember Chem 1104 experiment determining Ag,
Pb2 and Hg2
5
Ksp AgBr- 7.7 x 10-13
from table
s2 Ksp
s 8.8 x 10-7 M
Br- 0.0010 M
Remembering solubility rules, NaBr is soluble
0.00
0.0010
Initial (M)
s
s
Change (M)
s
0.0010 s
Equilibrium (M)
Ag s
Br- 0.0010 s ? 0.0010
s ltlt 0.0010
Ksp AgBr- 0.0010 x s
s 7.7 x 10-10 M
6
pH and Solubility
remove
add
At pH less than 10.45
Ksp MgOH- 2
Lower OH- / decrease pH
Ksp (s)(2s)2
Ksp 4s3
Ksp 8.9 x 10-12
Increase solubility of Mg(OH)2
At pH greater than 10.45
OH- 2s 2.8 x 10-4 M
pOH 3.55 pH 10.45
Raise OH- / increase pH
Le Chateliers Principle
Decrease solubility of Mg(OH)2
7
pH and Solubility
remove
add
As pH is lowered, H increases and can react
with SO42-
Ksp Ag2SO42- 1.4 x 10-5
Ksp (2s)2 (s)
Ksp 4s3
The system generates more SO42-, by dissolving
solid, to reestablish equilibrium.
s 0.015 M
SO42- and HSO4- are different species
Lower pH gt greater solubility
Raise OH- / increase pH
Decrease solubility of Ag2(SO4)
8
Complex Ion Equilibria and Solubility
A complex ion is an ion containing a central
metal cation bonded to one or more molecules or
ions.
The formation constant or stability constant (Kf)
is the equilibrium constant for the complex ion
formation.
16.10