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Chapter 17
Aqueous Equilibria
Sections 4 – 7
Adapted from:
Chemistry, The Central Science, 10th edition
Theodore L. Brown; H. Eugene LeMay, Jr.; and
Bruce E. Bursten
John D. Bookstaver
St. Charles Community College
St. Peters, MO
 2006, Prentice Hall, Inc.
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Solubility Products
Consider the equilibrium that exists in a
saturated solution of BaSO4 in water:
BaSO4(s)
Ba2+(aq) + SO42−(aq)
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Solubility Products
The equilibrium constant expression for
this equilibrium is
Ksp = [Ba2+] [SO42−]
where the equilibrium constant, Ksp, is
called the solubility product.
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Solubility Products
Ksp is not the same as solubility.
Solubility is generally expressed:
g/
L
g/
mL
mol/
L
(M)
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Questions
What is the expression for the Ksp of:
CaF2 (s) ↔ Ca2+ (aq) + 2F- (aq)
If the Ksp = 3.9 X 10-11, and [Ca2+] is
0.05 M what is [F-]?
ANS:2.8X10-5
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Question
What is the Ksp of a solution of Ag2CrO4
that has [Ag+] = 1.3 X 10-4 M?
ANS: 1.1 X 10-12
What is the Ksp of a solution of AgCl
that has [Ag+] = 1.35 X 10-5 M?
ANS: 1.82 X 10-10
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Factors Affecting Solubility
The Common-Ion Effect – Follow Le
Chatlier’s Principle!
BaSO4(s)
Ba2+(aq) + SO42−(aq)
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Questions
What is the molar solubility of CaF2 at 298 K in a
solution that is 0.010 M Ca(NO3)2? (NOTE: Ksp =
3.9 X 10-11)
ANS: 3.12 X 10-5 M
What would it be in 0.010 M NaF?
ANS: 3.9 X 10-7 M
What would be the M solubility of Mn(OH)2 in 0.020
M NaOH if the Ksp = 1.6 X 10-13
ANS: 4 X 10-10 M
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Factors Affecting Solubility
pH – Le Chatlier!
If a substance has a basic anion, it
will be more soluble in an acidic
solution.
e.g. NaF → Na+ + F- (add H+)
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Factors Affecting Solubility
pH – Le Chatlier!
Substances with acidic cations are
more soluble in basic solutions.
e.g. FeCl3 → Fe3+ + 3Cl- (add OH-)
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Questions
What sort of solution (acid/base/neither) will
each of the following be most soluble in?
Ni(OH)2
CaCO3
BaSO4
AgCl
CuS
NaSO3
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Factors Affecting Solubility
Complex Ions
Metal ions can act as
Lewis acids and
form complex ions
with Lewis bases in
the solvent.
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Complex Ions – La Chat!
The formation of these
complex ions increases
the solubility salts.
They have their own
special K called Kf.
The larger. The more
likely they’ll form.
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Factors Affecting Solubility
Amphoterism
Amphoteric metal oxides and
hydroxides are soluble in strong
acid or base, because they can act
either as acids or bases. (note:
ions are soluble in polar water)
e.g. Al3+, Zn2+, and Sn2+
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Example
Al3+
is really Al(H2O)63+
Al(H2O)63+(aq) + OH-(aq) →
Al(H2O)5(OH)2+(aq) + H2O(l)
Al(H2O)5(OH)2+(aq) + OH-(aq) →
Al(H2O)4(OH)2+(aq) + H2O(l)
+
Al(H2O)4(OH) (aq) + OH-(aq) →
Al(H2O)3(OH)3(s) + H2O(l)
Al(H2O)3(OH)3(aq) + OH-(aq) →
Al(H2O)2(OH)4-(aq) + H2O(l)
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Will a Precipitate Form?
In a solution,
If Q = Ksp, the system is at equilibrium
and saturated.
If Q < Ksp, more solid will dissolve until Q
= Ksp.
If Q > Ksp, the salt will precipitate until Q =
Ksp.
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Question
Will a precipitate form when 0.10 L of
8.0 X 10-3 M Pb(NO3)2 is added to
0.40 L of 5.0 X 10-3 M Na2SO4?
(Note: Ksp = 6.3 X 10-7 for PbSO4)
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Selective Precipitation of Ions
One can use
differences in
solubilities of salts
to separate ions in
a mixture.
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