2. Solubility
• The solubility of a substance is the amount of that
substance that will dissolve in a given amount of
solvent. “Solubility” may be considered to be an
equilibrium; the equilibrium is between solid and
ions in solution. Any ionic solid is 100% ionized in
aqueous solution; once it actually dissolves.
NaCl(s)
Na+
(aq) + Cl-
(aq)
3. Factors that affect solubility
• Temperature
– Solubility generally increases with temperature.
• Common ion effect
– Common ions reduce solubility
• pH of solution
– pH affects the solubility of ionic compounds.
• Formation of complex ion
– The formation of complex ion increases solubility
4. Solubility product
In general, solubility product, Ksp, is the mathematical
product of its dissolved ion concentrations raised to the
power of their stoichiometric coefficients. This
statement is called the solubility product principle
MyXz (s) yMZ+
(aq) + zXY-
(aq)
[ ] [ ]zyyz
sp
XMK −+
=
Solubility product constant Molar solubility of the ions
5. Solubility product
Solubility product constant (Ksp)
Most salts dissociate into ions when they dissolve.
For example:
BaSO4(s) ⇌ Ba2+
(aq) + SO4
2-
(aq)
This equilibrium system may be described by the
mass-action expression
Ksp = [Ba2+
][SO4
2-
]
6. BaSO4(s) ⇌ Ba2+
(aq)+ SO4
2-
(aq)
• The Ba2+
concentration and the SO4
2–
concentration
are equal since for each BaSO4 unit that dissolves
one Ba2+
and one SO4
2–
ion form.
Solubility = s = [Ba2+
] = [SO4
2–
]
From the above equation we know that
Ksp = [Ba2+
] [SO4
2–
]
Ksp = s × s = s2
Ksp = [Ba2+
] [SO4
2-
]
solubilityproduct
7. Problems on solubility product
• Calculate the Ksp value of BaSO4 which has a
solubility of 3.9×10 5‒
mol/L at 25°C.
BaSO4(s) ⇌ Ba2+
(aq)+ SO4
2-
(aq)
Ksp = [Ba2+
] [SO4
2–
]
Ksp = s × s = s2
= 3.9×10 5‒
× 3.9×10 5‒
= 1.52×10 9‒
8. Solubility and Solubility Products
(1) What is the solubility of AgCl if the Ksp is 1.6 x 10-10
AgCl(s) ⇌ Ag+
(aq) + Cl-
(aq)
Ksp = [Ag+
][Cl-
]
If s is the solubility of AgCl, then:
[Ag+
] = s and [Cl-
] = s
Ksp = (s)(s) = s2
= 1.6 x 10-10
s =
s = 1.3 x 10-5
mol/L
-10
10x1.6
9. • What is the solubility of CuS if the Ksp is 1.27× 10–45
.
CuS ⇌ Cu2+
+ S2–
Ksp = [Cu2+
] [S2–
]
= [s] [s]
= s2
s =
=
= 3.56 × 10–23
M
spK
-45
10x1.27
10. (1) What is the solubility of Ca(OH)2 if the Ksp is 7.9 x 10-6
Ca(OH)2 (s) Ca2+
(aq) + 2OH-
(aq)
1×s 2×s
[ ] [ ]212
sp OHCaK −+
=
[ ] [ ]21
sp 21K ss ××=
)2()2()1(Ksp sss ××=
)()221(Ksp sss ×××××=
3
sp 4K s=
12. Solubility and Solubility Products
What is the solubility of Mg(OH)2 if the Ksp is 1.6 x 10-10
Mg(OH)2(s) ⇌ Mg2+
(aq) + 2 OH-
(aq)
Ksp = [Mg2+
][OH-
]2
= 8.9 x 10-12
If the solubility of Mg(OH)2 is s mol/L, then:
[Mg2+
] = s mol/L and [OH-
] = 2s mol/L,
Ksp = (s)(2s)2
= 4s3
= 8.9 x 10-12
s = 1.3 x 10-4
mol/L
13. Different types of solution
Unsaturated solution: More solute can be
dissolved in it.
Saturated solution: No more solute can be
dissolved in it. Any more of solute you add
will not dissolve. It will precipitate out.
Super saturated solution: Has more solute
than can be dissolved in it. The solute
precipitates out.
