Chapter 18
The Solubility Product Constant
Write the net ionic equation
• Solutions of sodium chromate and barium
chloride are mixed.
Ba2+ + CrO42- → BaCrO4
Ba2+ + CrO42- ↔ BaCrO4
BaCrO4 ↔ Ba2+ + CrO42• Since the reaction is reversible when can flip it.
• Write the equilibrium expression for this reaction.
BaCrO4 ↔ Ba2+ + CrO42K = [Ba2+] [CrO42-]
BaCrO4 ↔ Ba2+ + CrO42Ksp = [Ba2+] [CrO42-]
• This is the solubility product expression
• It is used to determine the concentrations of
ions in a saturated solution of a slightly
soluble salt and determine whether a
precipitate will form within a solution.
Write the solubility product
expression for AgCl
Ksp = [Ag+] [Cl-]
Write the solubility product
expression for MgF2
Ksp = [Mg2+] [F-]2
Write the solubility product
expression for Ag3PO4
Ksp = [Ag+]3 [PO43-]
Molar Solubility
• Molar solubility is the maximum solubility of
a substance expressed in moles per liter.
• Example: The molar solubility of silver
chromate (Ag2CrO4) is 1.3 x 10-4 mol/L
The molar solubility of Hg2Cl2 is 6.5 x 10-7 mol/L.
Find the solubility product.
Ksp = 1.1 x 10-18
The molar solubility of Ag2CrO4 is 1.3 x 10-4 mol/L.
Find the solubility product.
Ksp = 8.8 x 10-12
The solubility of PbCrO4 is 4.30 x 10-5 g/L.
Find the solubility product.
What’s the difference here?
The solubility of PbCrO4 is 4.30 x 10-5 g/L.
Find the solubility product.
Ksp = 1.8 x 10-14
The solubility of PbCrO4 is 4.30 x 10-5 g/L.
Find the solubility product.
Ksp = 1.8 x 10-14
See Appendix D (Pages A6 & A7)
What are the ion concentrations?
What is the molar solubility PbCrO4?
Ksp of PbCrO4 = 1.8 x 10-14
Molar solubility = 1.3 x 10-7 mol PbCrO4/L
[Pb2+] = [CrO42-] = 1.3 x 10-7 M
What are the ion concentrations?
What is the molar solubility Ag2CrO4?
Ksp of Ag2CrO4 = 8.8 x 10-12
Molar solubility = 1.3 x 10-4 mol Ag2CrO4/L
[Ag+] = 2.6 x 10-4 M
[CrO42-] = 1.3 x 10-4 M
What are the ion concentrations?
What is the molar solubility Ag3PO4?
Ksp of Ag3PO4 = 1.8 x 10-18
Molar solubility = 1.6 x 10-5 mol Ag3PO4/L
[Ag+] = 4.8 x 10-5 M
[PO43-] = 1.6 x 10-5 M
What are the ion concentrations?
What is the molar solubility Pb3(PO4)2?
Ksp of Pb3(PO4)2 = 3.0 x 10-44
Molar solubility = 7.7 x 10-10 mol Pb3(PO4)/L
[Pb2+] = 2.3 x 10-9M
[PO43-] = 1.5 x 10-9 M
I place a large scoopful of Ag3PO4 (a white crystalline
solid) in a beaker of water and stir. I come back tomorrow.
What do I see in the beaker?
Did any of the Ag3PO4 dissolve?
What are the concentrations in the solution?
What is the molar solubility Ag3PO4?
Ksp of Ag3PO4 = 1.8 x 10-18
What are the ion concentrations?
 Molar solubility = 1.6 x 10-5 mol Ag3PO4/L
 [Ag+] = 4.8 x 10-5 mol Ag+/L
 [PO43-] = 1.6 x 10-5 mol PO43-/L
Did any of the Ag3PO4 dissolve? If any does dissolve
what are the concentrations in the solution?
 [Ag+] = 4.8 x 10-5M
 [PO43-] = 1.6 x 10-5M
How would you describe this solution?
Saturated Solutions and Solubility
Crystallization: opposite of the solution
process (solute becomes reattached to
surface of crystal)
Solute + Solvent
dissolve




crystallize
Solution
Saturated solution: no more solid will dissolve and
a dynamic equilibrium exists between the solution
and the undissolved solid.
What would we observe if we added sodium
phosphate solution to a beaker saturated
solution of Ag3PO4?
What would we observe if we added sodium
phosphate solution to the beaker containing a
saturated solution of Ag3PO4?
Ksp of Ag3PO4 = 1.8 x 10-18
[Ag+] = 4.8 x 10-5 mol Ag+/L
[PO43-] = 1.6 x 10-5 mol PO43-/L
[Ag+]3[PO43-]
= [4.8 x 10-5]3 [1.6 x 10-5]
=?
Why does Ag3PO4 precipitate?
Q
vs
K
sp
Reaction Quotient
If Q = Ksp, the system is at equilibrium
and the solution is saturated
If Q < Ksp, the solution is not saturated
precipitate does not form
If Q > Ksp, the solution exceeds
saturation and a precipitate forms
Does AgCl precipitate when equal volumes of
1.5 x 10-5M solutions of AgNO3 and NaCl mix?
Does AgCl precipitate when equal volumes of
1.5 x 10-5M solutions of AgNO3 and NaCl mix?
NO
Does AgCl precipitate when equal volumes of
1.5 x 10-5M solutions of AgNO3 and NaCl mix?
200.0ml of 0.0015M MgCl2 is mixed with 100.0ml
of 0.0015M NaOH. Does Mg(OH)2 precipitate?
200.0ml of 0.0015M MgCl2 is mixed with 100.0ml
of 0.0015M NaOH. Does Mg(OH)2 precipitate?
200.0ml of 0.0015M MgCl2 is mixed with 100.0ml
of 0.0015M NaOH. Does Mg(OH)2 precipitate?
AgNO3 solution is added to a solution of Na2CrO4.
What precipitate forms?
AgNO3 is added to a 0.0010M solution of Na2CrO4. What is
the [Ag+] when the Ag2CrO4 precipitate begins to form?
Clothing washed in water that
has a manganese
concentration exceeding 0.1
mg L-1 (1.8 x 10-6 M) may be
stained with manganese. A
laundry wishes to add a base
to precipitate manganese as
the hydroxide Mn(OH)2 (Ksp =
4.5 x 10-14). At what pH is
[Mn2+] equal to 1.8 x 10-6 M?
Clothing washed in water that has a manganese
concentration exceeding 0.1 mg L-1 (1.8 x 10-6 M) may be
stained with manganese. A laundry wishes to add a base to
precipitate manganese as the hydroxide Mn(OH)2 (Ksp = 4.5
x 10-14). At what pH is [Mn2+] equal to 1.8 x 10-6 M?
Fractional Precipitation
• The process by which two aqueous
substances in a solution are separated
through the addition of a common ion,
taking advantage of their different
concentration needs (Ksp values) in order
to form a precipitate.
smaller Ksp will
• The ion with the ________
precipitate first.
A solution contains 0.010 mol of KI and 0.10 mol of KCl
per liter. AgNO3 is gradually added to this solution.
Which precipitate forms first, AgCl or AgI?
• This is a multiple
equilibria problem.
• Strategy: If there is
more than one
equilibrium involved
write both and solve for
what you can.
A solution contains 0.010 mol of KI and 0.10 mol of KCl
per liter. AgNO3 is gradually added to this solution.
Which precipitate forms first, AgCl or AgI?
A solution contains 0.010 mol of KI and 0.10 mol of KCl
per liter. AgNO3 is gradually added to this solution. What
is the [I ̄ ] in the solution when AgCl starts to precipitate?
A solution contains 0.010 mol of KI and 0.10 mol of KCl per liter.
AgNO3 is gradually added to this solution. What percent of the
original [I ̄ ] remains in solution when AgCl starts to precipitate?
NH4Cl is added to a 750mL solution that is 0.10M in Mg2+ and
0.10M in NH3. The solution also contains a Mg(OH)2 precipitate.
Calculate the [NH4+] that is required to dissolve the precipitate.
NH4Cl is added to a 750mL solution that is 0.10M in Mg2+ and
0.10M in NH3. The solution also contains a Mg(OH)2 precipitate.
How many grams of NH4Cl were added?
We can increase or decrease the solubility of
“insoluble” (slightly soluble) substances by
applying LeChatlier’s Principle.
• The Common Ion Effect
• pH
The Common-Ion Effect
• A salt is less soluble in a solution that has
an ion in common with the salt.
• Calcium Sulfate (CaSO4) is “insoluble”
(slightly soluble) in water.
• Write the equation that represents the
solubility of calcium sulfate in water.
CaSO4(s) ⇌ Ca2+(aq) + SO42−(aq)
The Common-Ion Effect
CaSO4(s)⇌Ca2+(aq)+SO42−(aq)
• Describe the equilibrium system of
Calcium Sulfate in water.
•The solution is saturated with
most of the calcium sulfate in
solid form.
•The [SO42−] in the solution is very
low and is equal to the [Ca2+].
The Common-Ion Effect
CaSO4(s)⇌Ca2+(aq)+SO42−(aq)
• Describe what would happen if a solution of
sodium sulfate were added to the beaker.
•The [SO42−] would increase
causing the reaction to shift to
the left reducing the solubility of
CaSO4 causing the it CaSO4 to
precipitate out of the solution.
pH
• The pH can affect the solubility of a solute in
two ways:
– through the common ion effect.
Al(OH)3 ↔ Al3+ + 3OH ̄
• Is Al(OH)3 more soluble at a lower pH,
higher pH, or does pH not affect the
solubility of Al(OH)3 ?
Confused?
• What happens to this equilibrium if a strong
base such as NaOH is added?
• What happens to this equilibrium if a strong
acid such as HCl is added?
pH
• The pH can affect the solubility of a solute in
two ways:
– through the common ion effect.
or
– when a salt contains a basic anion such as F–,
CH3CO2–, or CN– it is normally more soluble at
low pH’s.
– When a salt contains an acidic ion such as Ag+,
Zn2+, or Al3+ it is often normally more soluble at
high pH’s.
FeS ↔ Fe2+ + S2• What happens to this equilibrium if a
strong acid such as HCl is added?
• A strong acid such as HCl will often
dissolve an insoluble salt by lowering the
concentration of the anion from the
solution and forming a weak acid.
AgCl ↔ Ag+ + Cl ̄
• What happens to this equilibrium if
ammonia or a strong base is added?
Adding Strong Bases or
Ammonia to Insoluble Salts
• A strong base or ammonia will often
dissolve an insoluble salt by lowering the
concentration of the cation from the
solution and forming a complex ion.
Hidden Slide
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