The Solubility Product Constant

1
Chapter 18

• The Solubility Product Constant

2
Review Quiz

• Nuclear Chemistry
• Thermochemistry
• Hesss Law
• Heats (Enthalpies) of

3
Write the net ionic equation

• Solutions of sodium chromate and barium chloride
  are mixed.

4
Ba2 CrO42- ? BaCrO4
5
Ba2 CrO42- ? BaCrO4
6
BaCrO4 ? Ba2 CrO42-

• Since the reaction is reversible when can flip
  it.
• Write the equilibrium expression for this
  reaction.

7
BaCrO4 ? Ba2 CrO42-

• K Ba2 CrO42-

8
BaCrO4 ? Ba2 CrO42-

• Ksp 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.

9
Write the solubility product expression for AgCl

• Ksp Ag Cl-

10
Write the solubility product expression for MgF2

• Ksp Mg2 F-2

11
Write the solubility product expression for Ag3PO4

• Ksp Ag3 PO43-

12
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

13
The molar solubility of Hg2Cl2 is 6.5 x 10-7
mol/L. Find the solubility product.

• Ksp 1.1 x 10-18

14
The molar solubility of Ag2CrO4 is 1.3 x 10-4
mol/L. Find the solubility product.

• Ksp 8.8 x 10-12

15
The solubility of PbCrO4 is 4.30 x 10-5 g/L.
Find the solubility product.

• Whats the difference here?

16
The solubility of PbCrO4 is 4.30 x 10-5 g/L.
Find the solubility product.

• Ksp 1.8 x 10-14

17
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)

18
What is the molar solubility PbCrO4?Ksp of
PbCrO4 1.8 x 10-14
What are the ion concentrations?

• Molar solubility 1.3 x 10-7 mol PbCrO4/L
• Pb2 CrO42- 1.3 x 10-7 M

19
What is the molar solubility Ag2CrO4?Ksp of
Ag2CrO4 8.8 x 10-12
What are the ion concentrations?

• Molar solubility 1.3 x 10-4 mol Ag2CrO4/L
• Ag 2.6 x 10-4 M CrO42- 1.3 x 10-4
  M

20
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 M PO43- 1.6 x 10-5
  M

21
What is the molar solubility Pb3(PO4)2?Ksp of
Pb3(PO4)2 3.0 x 10-44
What are the ion concentrations?

• Molar solubility 7.7 x 10-10 mol Pb3(PO4)/L
• Pb2 2.3 x 10-9M PO43- 1.5 x 10-9
  M

22
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?
23
Did any of the Ag3PO4 dissolve?
What are the concentrations in the solution?
24
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

25
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

26
How would you describe this solution?
27
Saturated Solutions and Solubility
Crystallization opposite of the solution
process (solute becomes reattached to surface
of crystal)
Saturated solution no more solid will dissolve
and a dynamic equilibrium exists between the
solution and the undissolved solid.
28
What would we observe if we added sodium
phosphate solution to a beaker saturated solution
of Ag3PO4?
29
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 Ag3PO43-
4.8 x 10-53 1.6 x 10-5 ? Why does
Ag3PO4 precipitate?
30
Q vs Ksp
Reaction Quotient

• If Q Ksp, the system is at equilibrium
• and the solution is saturated
• If Q lt Ksp, the solution is not saturated
• precipitate does not form
• If Q gt Ksp, the solution exceeds
• saturation and a precipitate forms

31
Does AgCl precipitate when equal volumes of 1.5 x
10-5M solutions of AgNO3 and NaCl mix?
?
?
32
Does AgCl precipitate when equal volumes of 1.5 x
10-5M solutions of AgNO3 and NaCl mix?NO
33
Does AgCl precipitate when equal volumes of 1.5 x
10-5M solutions of AgNO3 and NaCl mix?
34
200.0ml of 0.0015M MgCl2 is mixed with 100.0ml of
0.0015M NaOH. Does Mg(OH)2 precipitate?
?
?
35
200.0ml of 0.0015M MgCl2 is mixed with 100.0ml of
0.0015M NaOH. Does Mg(OH)2 precipitate?
Yes
36
200.0ml of 0.0015M MgCl2 is mixed with 100.0ml of
0.0015M NaOH. Does Mg(OH)2 precipitate?
37
AgNO3 solution is added to a solution of Na2CrO4.
What precipitate forms?
38
AgNO3 is added to a 0.0010M solution of Na2CrO4.
What is the Ag when the Ag2CrO4 precipitate
begins to form?
39
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?
40
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?
41
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.
• The ion with the ________ Ksp will precipitate
  first.

smaller
42
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.

43
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?
44
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?
45
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?
46
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.
47
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?
48
We can increase or decrease the solubility of
insoluble (slightly soluble) substances by
applying LeChatliers Principle.

• The Common Ion Effect
• pH

49
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)

50
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.

51
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.

52
pH

• The pH can affect the solubility of a solute in
  two ways
• through the common ion effect.

53
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?

54
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 pHs.
• When a salt contains an acidic ion such as Ag,
  Zn2, or Al3 it is often normally more soluble
  at high pHs.

55
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.

56
AgCl ? Ag Cl

• What happens to this equilibrium if ammonia or a
  strong base is added?

57
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.

58
Hidden Slide

• Remaining slides are hidden

59
Demo the previous slide

• Form AgCl precipitate by adding solutions of NaCl
  and AgNO3.
• Have concentrated solutions of NaOH and NH3
  available. Have students chose which they would
  like to see added to dissolve the precipitate.

60
Calculate the pH required to prevent the
precipitation of ZnS in a 0.050 M ZnCl2 solution
that is saturated with H2S (0.10M H2S).
61
Calculate the pH required to prevent the
precipitation of ZnS in a 0.050 M ZnCl2 solution
that is saturated with H2S (0.10M H2S).