SOLUBILITY EQUILIBRIA
SOLUBILITY
Solubility: quantity of a substance that dissolves to
form a saturated solution
Solubility: g/L
Molar solubility: mole/L
Precipitation
Dissolution
When forward rate = backward rate:
We have an _______________!
Some salts are very soluble (> 0.1 M). Recall
solubility rules.
Some salts are sparingly soluble (< 0.1 M)
sometimes referred to as ‘insoluble’.
AgCl(s)
Ag+(aq) + Cl!(aq)
Precipitation and Solubility of ionic salts and their
equilibrium in water
MA(s)
M+(aq) + A!(aq)
The concentration of solid does not
change at equilibrium
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SOLUBILITY CALCULATION
SOLUBILITY
Calculate [Ca2+] and [F-] for a saturated
CaF2 solution.
1) Write the reaction:
CaF2 (s)
Ca+2 (aq) +2F! (aq)
Solubility Product: Ksp
AgCl
AgBr
AgI
CdS
ZnS
Mg(OH)2
Ca(OH)2
CaF2
BaCO3
BaSO4
1.8 x 10-10
5.0 x 10-13
8.3 x 10-17
8.0 x 10-27
1.1 x 10-21
1.8 x 10-11
5.5 x 10-6
3.9 x 10-11
5.1 x 10-9
1.1 x 10-10
2) Make an ice table
3) Write the equilibrium expression
Ksp = [Ca2+][F!]2 =3.9 x 10!11 at 25oC
4) What is the solubility?
solubility = amount of salt dissociated
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SOLUBILITY CALCULATION
Common Ion Effect
Solubility and the
COMMON ION EFFECT
AgCl(s)
What is the solubility of CaF2 in 0.01 M
NaF solution? Ksp = 3.9 x 10!11
Ag+(aq) + Cl!(aq)
CaF2 solubility = [Ca2+] = x
[F!] = 0.01 M
I
C
E
CaF2(s)
Ca2+(aq) + 2F!(aq)
Add [Ag+] to solution at equilibrium
Add [Cl!] to solution at equilibrium
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Effect of pH on Solubility
Effect of pH on Solubility
What is the solubility of Mg(OH)2 in
water? Ksp = 1.8 x 10-11
What is the solubility of Mg(OH)2 in a
solution with a pH of 9? Ksp = 1.8 x 10-11
(Buffered to remain at this pH)
Mg(OH)2
Mg(OH)2 (s)
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Demo on Effect of pH on
Solubility
Demo on Effect of pH on
Solubility
1. Milk of Magnesia Mg(OH)2
MgCl2 + 2NaOH " Mg(OH)2(s) + 2NaCl
Mg(OH)2 (s)
3. Barium salts (continued)
BaCl2 + 2 Na2CO3 " BaCO3(s) + 2NaCl
BaCO3(s)
Ba+2(aq) + CO32!(aq)
CO32!(aq) + H2O
HCO3-(aq) + OH-(aq)
2. Barium salts
BaCl2 + 2NaOH " Ba(OH)2(s) + 2OH!
Ba(OH)2 (s)
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Demo on Effect of pH on
Solubility
Effect of pH on common ions
If either the anion or the cation is involved in an
acid base equilibrium, then it is a common ion
problem.
Barium salts (continued)
BaCl2 + 2 Na2SO4 " BaSO4(s) + 2NaCl
Ba+2(aq)
BaSO4(s)
+ SO4
2!(aq)
Basic Metal hydroxides
Example Mg(OH)2
Does Sulfate anion also react with water?
SO42!(aq) + H2O
Salts of weakly basic anions
Examples:
HSO4-(aq) + OH-(aq)
ZnCO3
NaF
What is a Basic anion? What about SO42-?
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Precipitation and Dissolution
AMPHOTERISM
Will Mn(OH)2 precipitate from 0.05 M
MnCl2 solution if the pH = 8?
Amphoterism is the ability of a molecule to
act both as an acid and as a base.
This results in metal hydroxides which are
soluble in both acids and bases.
Ksp = [Mn2+][OH!]2 = 1.9 x 10!13
1) MnCl2 (s)
Hydration: Al3+(aq) # Al(H2O)63+
2) If pH = 8, what is [OH-]?
Al(OH)3(s) + 3H2O
3) Are we at equilibrium?
In acid:
Al(OH)3(OH2)3 + H+
Al(OH)3(OH2)3(s)
Al(OH)2(OH2)4+(aq)
solubility increases.
In base:
Al(OH)3(OH2)3 + OH!
Al(OH)4(OH2)2!(aq)
+ H2O
solubility increases.
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AMPHOTERIC METAL
HYDROXIDES
FORMATION OF COMPLEX IONS
Hydration of metal ions
There are amphoteric hydroxides of
Al3+ Cr3+ Zn2+ Sn2+
Many transition metal ions
Cu2+(aq) + 4 H2O(l) "
Lewis
Acid
These involve formation of complex ions,
such as:
Al(OH)3(OH2)3
Metal Complex
Cu2+(aq) #
+ H+ " Al(OH)2(OH2)4+
+ OH! " Al(OH)4(OH2)2!
Other Lewis bases react with metal ions to
form complexes
Amphoteric hydroxides
Low and high pH increases solubility
Al(OH)3
Cr(OH)3
Zn(OH)3
Sn(OH)2 other transition metal hydroxides
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+ Lewis "
Base
Cu2+(aq) + 4 NH3(aq)
Cu2+(aq) + 4 CN!(aq)
Cu2+(aq) + 4 Cl!(aq)
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Problem Solving:
METAL COMPLEX STABILITY
Metal Complexes
Cu(OH2)42+ + 4NH3
Cu(NH3)42+ + 4H2O
[H2O] = constant
Keq =
Addition of 5 x10-3 total moles of Cu2+ to a
1.0 L solution of NaCN gives a solution of
the complex ion [Cu(CN)4]2! (Kf = 1 x 1025).
What is the concentration of uncomplexed
Cu2+ ion if the concentration of cyanide ion
in the final solution is 0.2 M?
Keq/[H2O]4 = Kf =
Kf VALUES OF METAL COMPLEXES
Ag(NH3)2+
1.7 x 107
Cu(NH3)42+
5 x 1012
Cu(CN)421 x 1025
Ag(CN)21 x 1021
Ag(S2O3)232.9 x 1013
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Cu2+(aq) +
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CN!(aq)
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[Cu(CN)4]! (aq)
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Applications of Complex Ion Formation
CuCO3 is a sparingly soluble salt:
Selective Precipitation of
metal ions
Add OH- to the following mixture of ions in solution;
0.05 M Mg2+
Ksp (CuCO3) = 2.3 x 10!10
0.02 M Cu2+
What is the solubility in water?
1) What [OH-] is necessary to begin precipitation of
each ion?
Ksp (Mg(OH)2) = 1.8 x 10-11
How can I get it to dissolve?
Ksp (Cu(OH)2) = 2.2 x 10-20
2) Which ion will precipitate first as OH- is added to the
solution?
Will it dissolve in ammonia solution?
What is Keq for the following reaction?
Mg2+ + 2OH-
Ksp CuCO3 = 2.3 x 10!10, Kf [Cu(NH3)4]2+ = 5 x 1012
CuCO3(s) + 4NH3(aq)
Mg(OH)2(s)
CO32!(aq) + [Cu(NH3)4]2+(aq)
Cu2+ + 2OH-
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Cu(OH)2(s)
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