CHM 1046: General Chemistry and
Qualitative Analysis
Unit 20
Aqueous Equilibria:
Solubility Product
Dr. Jorge L. Alonso
Miami-Dade College
– Kendall Campus
Miami, FL
Textbook Reference:
•Chapter # 20
•Module # 10
Aqueous
Equilibria
Cation Analysis
Acid pH
Basic
SOLUBILITY RULES:
1. All salts of alkali metals (IA) are
soluble.
2. All NH4+ salts are soluble.
3. All salts containing the anions: NO3-,
ClO3-, ClO4-, (C2H3O2-) are soluble.
4. All Cl-, Br-, and I- are soluble except for
Ag+, Pb2+, and Hg22+ salts.
5. All SO42- are soluble except for Pb2+,
Sr2+, and Ba2+.
6. All O2- are insoluble except for IA
metals Ca2+, Sr2+, and Ba2+ salts.
7. All OH- are insoluble except for IA
metals, NH4+ & slightly soluble Ca 2+
Ba2+ & Sr2+
6. All salts containing the anions: CO32-,
PO43-, AsO43-, S2- and SO32- are
insoluble except fro IA metals and
NH4+ salts.
7. For salts containing the anions not
mentioned above (e.g., CrO42-,
Cr2O72-, P3-, C2O42- etc.) assume that
Aqueous
they are insoluble except for
IA
Equilibria
+
metals and NH4 salts, unless,
otherwise informed.
Precipitation Reactions
• These reactions are governed by the basic
principles of the solubility rules:
AgNO3(aq) + NaCl(aq)  AgCl (s) + NaNO3 (aq)
+ NaI(aq)
{AgNO3+NaCl&NaI}
Aqueous
Equilibria
Solubility Product
Does BaSO4 dissolve in water?
H2O
BaSO4(s)
Ba2+(aq) + SO42−(aq)
This equilibrium is that exists in a saturated solution of BaSO4 in water has
an equilibrium constant expression:
Ksp = [Ba2+] [SO42−] = 1.1 x 10 -10
where the equilibrium constant, Ksp, is called the solubility product.
Aqueous
Equilibria
o
Table of Selected Solubility Products Constants at 25 C
Compound
aluminum hydroxide
aluminum phosphate
Formula
Al(OH)3
AlPO4
Ksp
4.6 x 10-33
6.3 x 10-19
barium carbonate
barium chromate
barium fluoride
BaCO3
BaCrO4
BaF2
5.1 x 10-9
2.2 x 10-10
1.0 x 10-6
barium hydroxide
barium iodate
barium oxalate
barium sulfate
Ba(OH)2
Ba(IO3)2
BaC2O4
BaSO4
5 x 10-3
1.5 x 10-9
2.3 x 10-8
1.1 x 10 -10
barium sulfite
barium thiosulfate
BaSO3
BaS2O3
8 x 10-7 Aqueous
Equilibria
1.6 10-5
*
Are Solubility Product (Ksp) and
Solubility the Same Things?
Ca(OH)2
0.823 g/L
 grams

Liter 

3
Ksp = [Ca2+] [OH−]2
 
-6  
= 5.5 x 10
L
Molarity
Molarity of Ions
2
 moles Ca(OH)2 




Ca
2
OH
M
&
 M 


Liters
of
Solution


Liter
Liter

 

Solubility: mostly used to
describe substances that
dissolve very well in water
Ca(OH)2
FW= 74 g/
Solubility Product: used
Aqueous
for substances do not
Equilibria
dissolve well in water
Calculating Solubility Product (Ksp)
from Solubility and vice versa
Ca(OH)2
0.823 g/L
Solubility of
Compound
 grams

Liter 

Ca(OH)2
FW= 74 g/
Ksp
Ksp = [Ca2+] [OH−]2
Molarity of
Compound
 moles Ca OH 2 
M

 Liters of Solution 
Molarity of Ions
1 Ca 2   2 OH- 
M 
&

 Liter   Liter 
 0.823 g  
  0.0111 M
L  74g 

[Ca2+] = 0.0111 M
= [Ca2+] [OH−]2
= (0.0111) (0.0222)2
[OH−] = 2(0.0111M)
Aqueous
= 5.5 x 10-6
= 0.0222Equilibria
M
Factors Affecting Solubility
•
•
•
•
•
Temperature
Common Ion Effect
pH
Complex Ions
Amphoterism
Aqueous
Equilibria
Factors Affecting Solubility:
(1)Temperature
Generally, the
solubility of solid
solutes in liquid
solvents increases
with increasing
temperature.
HO
2
NaCl
Na+(aq) + Cl-(aq)
+ Heat
ΔHsoln = + 3.80 kJ/η
H2O
2Ce3+(aq) + 3SO42+Aqueous
Heat
Equilibria
ΔHsoln = - 17 kJ/η
Ce2(SO4)3
Factors Affecting Solubility:
(2) The Common-Ion Effect
 If one of the ions in a solution equilibrium is added to
a solution containing that ion, the equilibrium will shift
to the left and the solubility of the salt will decrease.
Na2SO4(s)
2Na+(aq) + SO42−(aq)
BaSO4(s)
Ba2+(aq) + SO42−(aq)
Ksp = [Ba2+] [SO42−] = 1.1 x 10 -10
Aqueous
Equilibria
Factors Affecting Solubility: (3) pH
{Milk of Magnesia}
Mg(OH)2 (s) ↔ Mg2+(aq) + 2 OH –(aq)
Ksp = 1.8 x 10-11
Mg(OH)2 (s) + 2 H+  Mg2+(aq) + 2 H2O(l)
 If a substance has a
basic anion, it will
be more soluble in
an acidic solution.
CaCO3(s) ↔ Ca2+(aq) + CO32-(aq)
CO32-(aq) + H+(aq)  HCO3-(aq)
CaCO3(s) + H+ (aq)  Ca2+(aq) + HCO3-(aq)
Ksp = 3.8 x 10-9
Aqueous
Equilibria
Complex Ions (Coordination Compounds)
Exhibit beautiful colors, are composed of two parts:
(1) Central
metal Ion
(usually a
transition metal
ion)
Function as
Lewis acids:
Co2+, Cu2+, Fe3+
2+
(2) Several
molecules &/or
ions (known
as Ligands)
Function as
Lewis bases:
Complex ions are
extremely soluble
Aqueous
Equilibria
Factors Affecting Solubility
pH and formation of Complex Ions
• Complex Ions
AgCl(s)
 The formation
of these
complex ions
increases the
solubility of
these salts.
Aqueous
Equilibria
Factors Affecting Solubility
(4) pH and formation of Complex Ions
Are any of the following substances soluble? Can they be made to dissolve?
Ag2O
AgOH
CdO
Cr2O3
CoO
CuO
NiO
FeO
Fe2O3
Cd(OH)2
Cr(OH)3
Co(OH)2
Cu(OH)2
Ni(OH)2
Fe(OH)2
Fe(OH)3
Solubility Rules:
6. All O2- are insoluble except for IA metals Ca2+, Sr2+, and Ba2+ salts.
7. All OH- are insoluble except for IA metals, NH4+ & slightly soluble Ca 2+ Ba2+ & Sr2+
Oxides and Hydroxides of these ions are fairly insoluble
But complex ions of these metals are extremely soluble.
Fe(OH)2 (s)
Fe2+(aq) +
2 OH- (aq) Not very soluble
Ksp = 8.0 x 10-16
Add Ligand:
Fe2+
+
6 CN-
Fe(CN)63- (aq)
Aqueous
ExtremelyEquilibria
soluble
Kf = 1.0 x 1042
Factors Affecting Solubility
pH and formation of Complex Ions
 Substances with acidic cations are more soluble in basic solutions.
 Metal ions can act as Lewis acids (e- pair acceptors) and form
complex ions with Lewis bases (e- pair donors) in the solvent.
(Formation Constants)
Lewis
Bases
OHNH3
CN-
SCNBrS2O32-
(thiosulfate)
Aqueous
Equilibria
Factors Affecting Solubility :
(5) Amphoterism
 Amphoteric metal oxides and hydroxides are soluble in strong acid or
base, because they can act either as acids or bases.
 {Amphiprotic can gain or loose electrons H+ + CO3- HCO3- H2CO3.}
 Examples of Amphoteric cations are Al3+, Zn2+, and Sn2+.
OH-
OH-
3+
Al
(aq)
Acid
H+
OH-
Al(OH)3 (s)
neutral ppt
OHH+
Aqueous
Al(OH)43-(aq) Equilibria
Base
Will a Precipitate Form?
Ba2+(aq) + SO42−(aq)
BaSO4(s)
At Equilibrium, Ksp = [Ba2+] [SO42−] = 1.1 x 10 -10
Problem: What is the maximum amount (grams) of BaSO4
(FW=233 g/) that will dissolve in water (saturated solution)?
Molarity of
Compound
Molarity of Ions
2  Ba 2   η SO4 
M
&


 Liter   Liter 
Ksp = [Ba2+] [SO42−]
= 1.1 x 10 -10
[Ba2+]
10
= 1.1 x 10

[Ba2+] = 1.05x10 L
5
Solubility of
Compound
 grams

Liter 

 moles BaSO 4 
M

 Liters of Solution 
[BaSO4] = 1.05x10
5

L
1.05x105   233 g  
L 


0.0025 g
L Aqueous
Equilibria
Will a Precipitate Form?
BaSO4(s)
Ba2+(aq) + SO42−(aq)
At Equilibrium, Ksp = [Ba2+] [SO42−] = 1.1 x 10 -10
At non-Equilibrium conditions, Q = [Ba2+] [SO42−]
• In a solution,
 If Q = Ksp, the system is at equilibrium
and the solution is saturated.
 If Q < Ksp,
more solid will dissolve until Q = Ksp.
 If Q > Ksp,
the salt will precipitate until Q = Ksp.
Aqueous
Equilibria
2001 Q1
Aqueous
Equilibria
At Equilibrium, Ksp = [Pb2+] [Cl−]2 = 1.6 x 10 -5
Aqueous
Equilibria
At Equilibrium, Ksp = [Ag+] [Cl−] = 1.8 x 10 -10
At Equilibrium, Ksp = [Pb2+] [Cl−]2 = 1.6 x 10 -5
Aqueous
Equilibria
2004A Q1
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Equilibria
Aqueous
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2002B Q5
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2006A Q1
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2002
B
Aqueous
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Aqueous
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2004
A
Aqueous
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2004 A
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2005
A
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2006
A
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2006 (A)
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2006 (A)
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Equilibria
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Aqueous
Equilibria
Formation Constants of Selected Complex Ions, at 25oC
Formula
Kf
Ag(CN)2-
5.6 x 1018
Co(SCN)+
100
Fe(SCN)2+
900
HgI42-
4.2 x 1027
Zn(NH3)42+
2.9 x 109
Zn(OH)42-
4.6 x 1017
Aqueous
Equilibria
Solubility of Chemical Substances
Elements: mostly insoluble solids, liquids & gases.
Covalent Compounds: mostly insoluble gases, except
O & N containing organic liquids (polar: acids, bases,
alcohols, etc.)
Ionic Compounds: many are soluble.
SOLUBILITY RULES: for Ionic Compounds (Salts)
1. All salts of alkali metals (IA) are soluble.
2. All NH4+ salts are soluble.
3. All salts containing the anions: NO3-, ClO3-, ClO4-, (C2H3O2-) are soluble.
4. All Cl-, Br-, and I- are soluble except for Ag+, Pb2+, and Hg22+ salts.
5. All SO42- are soluble except for Pb2+, Sr2+, and Ba2+.
6. All O2- are insoluble except for IA metals Ca2+, Sr2+, and Ba2+ salts.
HO
{Soluble metal oxides form hydroxides: CaO 2
Ca 2+ + 2OH-}
7.
6.
7.
All OH- are insoluble except for IA metals, NH4+ & slightly soluble Ca 2+ Ba2+ & Sr2+
All salts containing the anions: CO32-, PO43-, AsO43-, S2- and SO32- are insoluble
except fro IA metals and NH4+ salts.
For salts containing the anions not mentioned above (e.g., CrO42-, Cr2O72-Aqueous
, P3-,
C2O42- etc.) assume that they are insoluble except for IA metals and NH4+ Equilibria
salts,
unless, otherwise informed.
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.
 Examples of
such cations
are Al3+,
Zn2+, and
Sn2+.
OH
-
OH-
Aqueous
Equilibria
Weak acid