Solubility
Equilibria
Will it all dissolve, and if
not, how much will?
SOLUBILITY EQUILIBRIA
• Solubility: Relative term used to describe how
much of a particular substance dissolves in a
certain amount of solvent.
• Substances that dissolve very well are said to
be soluble
• Insoluble species don’t dissolve well.
• All substances are “soluble” to some extent
• We will look at slightly soluble substances
SOLUBILITY EQUILIBRIA
• All dissolving is an equilibrium.
• If there is not much solid it will
all dissolve.
• As more solid is added the
solution will become saturated.
• Solid ↔ dissolved
• The solid will precipitate as fast
as it dissolves, forming an
equilibrium.
Watch out
• Solubility is not the same
as solubility product.
• Solubility product is an
equilibrium constant.
• It doesn’t change except
with temperature.
• Solubility is an equilibrium
position for how much can
dissolve.
• A common ion can change
this.
Ksp Values for Some Salts at 25C
Name
Formula
Ksp
Barium carbonate
BaCO3
2.6 x 10-9
Barium chromate
BaCrO4
Barium sulfate
Formula
Ksp
Lead(II) bromide
PbBr2
6.6 x 10-6
1.2 x 10-10
Lead(II) chloride
PbCl2
1.2 x 10-5
BaSO4
1.1 x 10-10
Lead(II) iodate
Pb(IO3)2
3.7 x 10-13
Calcium carbonate
CaCO3
5.0 x 10-9
Lead(II) iodide
PbI2
8.5 x 10-9
Calcium oxalate
CaC2O4
2.3 x 10-9
Lead(II) sulfate
PbSO4
1.8 x 10-8
Calcium sulfate
CaSO4
7.1 x 10-5
Magnesium carbonate
MgCO3
6.8 x 10-6
Copper(I) iodide
CuI
1.3 x 10-12
Magnesium hydroxide
Mg(OH)2
5.6 x 10-12
Copper(II) iodate
Cu(IO3)2
6.9 x 10-8
Silver bromate
AgBrO3
5.3 x 10-5
Copper(II) sulfide
CuS
6.0 x 10-37
Silver bromide
AgBr
5.4 x 10-13
Iron(II) hydroxide
Fe(OH)2
4.9 x 10-17
Silver carbonate
Ag2CO3
8.5 x 10-12
FeS
6.0 x 10-19
Silver chloride
AgCl
1.8 x 10-10
Fe(OH)3
2.6 x 10-39
Silver chromate
Ag2CrO4
1.1 x 10-12
Lead(II) bromide
PbBr2
6.6 x 10-6
Silver iodate
AgIO3
3.2 x 10-8
Lead(II) chloride
PbCl2
1.2 x 10-5
Silver iodide
AgI
8.5 x 10-17
Lead(II) iodate
Pb(IO3)2
3.7 x 10-13
Strontium carbonate
SrCO3
5.6 x 10-10
Lead(II) iodide
PbI2
8.5 x 10-9
Strontium fluoride
SrF2
4.3 x 10-9
Lead(II) sulfate
PbSO4
1.8 x 10-8
Strontium sulfate
SrSO4
3.4 x 10-7
ZnS
2.0 x 10-25
Iron(II) sulfide
Iron(III) hydroxide
Name
Zinc sulfide
SOLUBILITY PRODUCT CONSTANTS
 Consider the following reaction
PbCl2(s)
2+
-
Pb (aq) + 2Cl (aq)
 The equilibrium constant expression is
Ksp = [Pb2+][Cl-]2
 Ksp is called the solubility product constant or
simply solubility product
 For a compound of general formula, MyXz
(next page)
z+
MyXz(s)
y-
yM (aq) + zX (aq)
Ksp = [Mz+]y[Xy-]z
Mg2+(aq) + NH4+(aq) + PO43-(aq)
MgNH4PO4(s)
Ksp = [Mg2+][NH4+][PO43-]
2+
Zn(OH)2(s)
-
Zn (aq) + 2OH (aq)
Ksp = [Zn2+][OH-]2
Ca3(PO4)2(s)
2+
3Ca (aq) +
Ksp = [Ca2+]3[PO43-]2
32PO4 (aq)
 Molar solubility: the number of moles that
dissolve to give 1 liter of saturated solution
 As with any equilibrium constant the numerical
value must be determined from experiment
 The Ksp expression is useful because it applies
to all saturated solutions
- the origins of the ions are not relevant
 Consider that @ 25C Ksp AgI = 1.5 x 10-16
Solving Solubility Problems
For the salt AgI at 25C, Ksp = 1.5 x 10-16
AgI(s)  Ag+(aq) + I-(aq)
I
O
O
C
+x
+x
E
x
x
1.5 x 10-16 = x2
x = solubility of AgI in mol/L = 1.2 x 10-8 M
Solving Solubility Problems
For the salt PbCl2 at 25C, Ksp = 1.6 x 10-5
PbCl2(s)  Pb2+(aq) + 2Cl-(aq)
I
O
O
C
+x
+2x
E
x
2x
1.6 x 10-5 = (x)(2x)2 = 4x3
x = solubility of PbCl2 in mol/L = 1.6 x 10-2 M
Relative Solubilities
• Ksp will only allow us to compare the
solubility of solids the that fall apart
into the same number of ions.
• The bigger the Ksp of those the more
soluble.
• If they fall apart into different number
of pieces you have to do the math.
The Common Ion Effect
• When the salt with the anion of a weak
acid is added to that acid:
– it reverses the dissociation of the acid.
– lowers the percent dissociation of the acid.
• The same principle applies to salts with
the cation of a weak base..
• The calculations are the same as with
acid base equilibrium.
Solving Solubility with a Common Ion
For the salt AgI at 25C, Ksp = 1.5 x 10-16
What is its solubility in 0.05 M NaI?
AgI(s)  Ag+(aq) + I-(aq)
I
O
0.05
C
+x
+x
E
x
0.05+x
1.5 x 10-16 = (x)(0.05+x)  (x)(0.05)
x = solubility of AgI in mol/L = 3.0 x 10-15 M
pH and solubility
• OH- can be a common ion.
• More soluble in acid.
• For other anions if they come from a
weak acid they are more soluble in a
acidic solution than in water.
• CaC2O4 ↔ Ca+2 + C2O4-2
• H+ + C2O4-2 ↔ HC2O4• Reduces C2O4-2 in acidic solution.
Precipitation
• The reaction quotient (called ion product)
may be applied to solubility equilibria determines if a substance will precipitate
from solution
• Ion Product, Q =[M+]a[Nm-]b
• If Ksp<Q a precipitate forms, reverse
process occurs
• If Ksp=Q equilibrium solution is just
saturated
• If Ksp>Q No precipitate, forward process
occurs
Precipitation Example
• A solution of 75.0 mL of 0.020 M BaCl2 is added
to 125.0 mL of 0.040 M Na2SO4. Will a
precipitate form? (Ksp= 1.5 x 10-9M BaSO4)
BaSO4 could form if Ksp<Q.
For Q you need initial concentrations:
[Ba2+] = mmol Ba2+ / total mL
= (75.0mL)(0.020 M)/(75.0mL + 125 mL) = 0.0075 M
[SO42-] = mmol SO42- / total mL
= (125.0mL)(0.040 M)/(75.0mL + 125 mL) = 0.025 M
Q = [Ba2+] [SO42-] = (0.0075 M)(0.025 M) = 1.9 x 10-4
Ksp<Q so BaSO4 will form.
To figure out concentrations set up an ice table.
Complex Ions
A Complex ion is a charged species composed
of:
1. A metallic cation
2. Ligands – Lewis bases that have a
lone electron pair that can form a
covalent bond with an empty orbital
belonging to the metallic cation
NH3, CN-, and H2O
are Common Ligands
N
H
H
H
-
C
N
O
H
H
The Addition Of Each Ligand
Has Its Own Equilibrium
• Usually the ligand is in large excess.
• And the individual K’s will be large so we
can treat them as if they go to
equilibrium.
• The complex ion will be the biggest ion
in solution.
Coordination Number
 Coordination number refers to the
number of ligands attached to the cation
 2, 4, and 6 are the most common
coordination numbers
Coordination
Example(s)
number
2
Ag(NH3)2+
4
CoCl42-
6
Co(H2O)62+
Cu(NH3)42+
Ni(NH3)62+
Complex Ions and Solubility
AgCl(s)  Ag+ + Cl-
Ksp = 1.6 x 10-10
Ag+ + NH3  Ag(NH3)+
K1 = 1.6 x 10-10
Ag(NH3)+ NH3  Ag(NH3)2+
AgCl + 2NH3  Ag(NH3)2+ + Cl-
K2 = 1.6 x 10-10
K = KspK1K2

3 2

[ Ag ( NH ) ][Cl ]
K  2.8 x 10 
2
[ NH 3 ]
3
Pamukkale is one of the
extraordinary natural
wonders of Turkey.
The great attraction is the
white immensity of the cliff
with sculptured basins full of
water and congealed
waterfalls; they seem done
of snow, cloud, cotton.
The scientific explanation is the hot thermal places that lie
under the mount provoke the calcium carbonate spill, that
makes the forms as solid as travertino marble.
One can bath there; the Turks call this place
PAMUKKALE, which means "Castle of Cotton“.
It is a protecting landscape that fascinates, as the action of
the mineral waters that contains calcium oxides left fantastic
marks in the structures.
The resultant effect is spectacular: the waters spill on a
series of steps, forming solid cascades and pools.
As much the cascades of calcium carbonate as the water
change color in accordance with changes of the solar light
that illuminates them, and the effect is surprising.
At times white, others blue, or green or
other colors. The spectacle is flaring.
The continuous dynamics of the erosion and the
transformation of the natural landscape result in an
unusual environment.
PAMUKKALE is one of the most unique phenomena in nature.