The Solubility Product Constant, Ksp
Recall that for any general reaction:
aA + bB
cC + dD
an equilibrium constant expression can be defined as:
RECALL: The concentrations of
solids and liquids are NOT included
in the expression because while
their amounts will change during a
reaction, their concentrations will
remain constant.
Since saturated solutions are equilibrium systems, we can apply this mathematical relationship to solutions.
We will refer to our equilibrium constant as Ksp, where "sp" stands for "solubility product"
For a silver sulfate saturated solution:
Ag2SO4 (s)
2Ag+(aq) + SO42-(aq)
we can write our solubility product constant expression as:
*Please note that it is VERY
IMPORTANT to include the
ion charges.
Unless otherwise indicated, our examples will always involve dissolving a solid to produce aqueous ions.
Ksp, which again is just a special case of Keq, provides us with the same useful information:


A low value of Ksp indicates a substance that has a low solubility (it is generally insoluble); for ionic compounds
this means that there will be few ions in solution.
Iron(II) sulfide, FeS, is an example of a low Ksp : Ksp = 4 ×10-19
In a saturated solution of FeS there would be few Fe2+ or S2- ions.
A large value of Ksp indicates a soluble substance; for ionic compounds it tells us that there will be many ions in
solution.
Lead(II) chloride, PbCl2 which has a Ksp of 1.8 ×10-4 is an example of a relatively large Ksp
A saturated solution of PbCl2 would have a relatively high concentration of Pb 2+ and Cl - ions.
Examples:
1) Write the expression for the solubility product constant, Ksp, for Ca3(PO4)2.
2) The concentration of a saturated solution of BaSO4 is 3.90 × 10-5M. Calculate Ksp for barium sulfate at 25°C.
3) The concentration of lead ions in a saturated solution of PbI 2 at 25°C is 1.3 × 10-3 M. What is its Ksp?
4) Ksp for MgCO3 at 25°C is 2.0 × 10-8. What are the ion concentrations in a saturated solution at this temperature?
5) Ksp for silver chromate, Ag2CrO4 , is 1.1 × 10-12. Calculate the molar concentration of silver chromate in a saturated
solution.
NAME ______________________ DATE __________ DUE DATE _______________
PART I: Assignment- The Solubility Product Constant, Ksp
1. What does Ksp stand for?
2. If Keq is very large, which concentration is larger: reactants or products?
3. If Keq is very small, which concentration is larger: reactants or products?
4. Why does the solubility product constant expression not include the concentration of the reactant in
the expression?
PART II: Assignment- The Solubility Product Constant, Ksp
1. Write the balanced equation and the solubility product constant expression, Ksp, for the each of the
following dissociation reactions. All compounds are solids. One has been given as an example.
An important reminder – the seven diatomic molecules (H2, N2, O2, F2, Cl2, Br2, I2) ARE NOT diatomic
as ions in solution!
Reminders
– ion charges MUST BE included.
– solids (and liquids) are NOT included in the equilibrium expression
– don’t forget to include exponents when needed
– polyatomic ions (e.g. CO3-) do not break apart
Equation
Compound
(NH4)2S
(NH4)2S (s)
2 NH4 +(aq) + S2-(aq)
Ksp
Ksp = [NH4+]2[S2-]
CaS
K2SO4
Mg(OH)2
2. Consider the Ksp values for the following substances, all measured at 25C
PbCrO4
2.0  10-16
PbSO4
1.3  10-8
PbCO3
7.4  10-14
a) Which substance is MOST soluble (dissolves the best)
_____________________
b) Which substances is LEAST soluble (dissolves the worst) _____________________
3. Solutions are prepared by dissolving a sulfate salt (an ionic compound containing the sulfate ion) in
water. Which one of the compounds listed would we use in order to get the highest
concentration of sulfate ion, SO42-?
(a salt is an ionic compound that contains a positive ion other than hydrogen and a negative ion other than
hydroxide).
A.
B.
C.
D.
CaSO4
SrSO4
BaSO4
RaSO4
Ksp
Ksp
Ksp
Ksp
= 2.6  10-4
= 7.6  10-7
= 1.5  10-9
= 4.0  10-11
4. Calculate Ksp for a saturated nickel(II) sulfide, NiS, solution with a molar concentration of
3.27  10-11
5. At 25C, the concentration of Ce(OH)3 in a saturated solution is 5.1  10-6 M. Calculate Ksp.
6. Calculate the concentration of ions in a saturated solution of CaCO3 in water at 25C. Ksp for
CaCO3 is 4.8  10-9.
7. Calculate the concentration of ions in a saturated solution of magnesium chlorate in water at
25C which has a Ksp of 4.5 x 10-5.