PP 25: Solubility Equilibria
Drill: Determine the Q-/HQ ratio to make a buffer solution with a pH of 4.70:
(Ka for HQ = 3.0 x 10-5)
Solubility: The amount of one substance that will dissolve in another
Saturated Solution: When the maximum amount of one substance is dissolved in another
Precipitation: When dissolved particles come together, change phases & fall out of the solution.
When more than the maximum amount of solute is added to a solution, a ppt will form
Precipitate (ppt): The new phase that forms and leaves the solution
• Water vapor combining to form rain
• Dissolved ions combining to form a solid
Solubility Equilibrium:
Write the equilibrium expression for the following reaction:
AgCl(s)  Ag+ + Cl-
Becaure the AgCl is a solid, not in phase with the solution, it counts as 1 in the Keq.
Equilibrium Expression: Ksp = [Ag+][Cl-]
MX(s)  M+(aq) + X-(aq)
Solubility Products: Rxn:
Equilibrium Expression: Ksp = [M+][X-]
MX2(s)  M+(aq) + 2 X-(aq)
Solubility Products: Rxn:
Equilibrium Expression: Ksp = [M+][X-]2
Problem: Write the equation & equilibrium expression for: Al2(SO4)3
Al2(SO4) 3(s)  2 Al+3(aq) + 2 SO4-2(aq)
Rxn:
Equilibrium Expression: Ksp = [Al+3]2[ SO4-2]3
Drill: Write the reaction & equilibrium expression when solid Fe2(SO4)3 is added to water.
Solubility Equilibria Calculations:
Demonstrate: Calculate the solubility of BaSO4(s)
1.
2.
3.
4.
5.
Set up & balance rxn:
Assign Eq. amounts:
Write the Keq:
Substitute:
Solve for x:
(Ksp = 1.2 x 10-10)
1 BaSO4  1 Ba+2 + 1 SO4-2
1–x
x
x
+2
-2
Ksp = [Ba ][ SO4 ]
Ksp = [x][x]
Ksp = [x]2 = Ksp = 1.2 x 10-10
x = solubility = 1.1 x 10-5 M
Problems:
 Calculate the solubility of SrCr2O7(s) :
 Calculate the solubility of PbI2(s):
 Calculate the solubility of Mn2S3(s)
(Ksp = 2.5 x 10-13)
(Ksp = 8.0 x 10-9)
(Ksp = 1 x 10-98)
Common Ion Effect: The solubility of any compound is greatly reduced if one of the ions that
make up the compound is already present in the solution.

Example: The solubility of PbCl2 is greatly reduced when NaCl is added to the solution.
Drill: Calculate the solubility & the maximum molarity of all ions in a solution of MZ3. (Ksp for MZ3 = 2.7 x 10-19)
Calculate Solubility of Each:



MQ2:
Ksp = 3.2 x 10-11
MZ3:
Ksp = 2.7 x 10-15
CaCO3:
Ksp = 4.4 x 10-16
Drill: Calculate the solubility of M3Q4
(Ksp for M3Q4= 6.9 x 10-207)
Solubility Concepts: Rxn: AgI(s)  Ag+ + I Ksp = [Ag+][I-] at Eq.
 Qsp = [Ag+][I-] init.
 As with equilibrium constants earlier, Q has the same formula, but refers to the start of a reaction rather
than equilibrium
 If Qsp > Ksp , a precipitate will form
 If Qsp < Ksp , no precipitate will form
Problem: 150 mL 0.0010 M KI is added to 100 mL 0.050 M Pb(NO3)2. (Ksp for PbI2 = 8.0 x 10-9)
• Will a ppt form?
Problem: 300.0 mL 0.0010 M KQ is added to 200.0 mL 0.050 M MX2. (Ksp for MQ2 = 6.0 x 10-9)
• Will a ppt form?
Drill: Calculate the solubility of AgI(s)
(Ksp = 1.5 x 10-16)
Common Ion Effect: Solubility of a solid is reduced when one of its ions is already in solution
Common Ion Solubility Calculation Demonstration:
• Calculate the solubility of AgI(s) in 0.10 M NaI :
1. Set up & balance rxns:
2. Assign Eq. amounts:
3. Write Keq:
4. Substitute:
5. Solve for x:
(Ksp = 1.5 x 10-16)
AgI(s)  Ag+ + I1–x
x
x
NaI(s)
 Na+ + I0.10 – all
0.10 0.10
Ksp = [Ag+][I-]
Ksp = [x][0.10 + x]
Ksp = 0.10x = 1.5 x 10-16
x = solubility = 1.5 x 10-16 M
Problems:
• Calculate the solubility of MZ2(s) in 0.20 M M(NO3)2:
(Ksp = 2.0 x 10-15)
• 30.0 mL 0.0010 M KBr is added to 20.0 mL 0.050 M Y(NO3)2. (Ksp for YBr2 = 6.0 x 10-9)
Will a ppt. form?
Drill: 10.0 mL 0.050 M KCl is added to 15 mL 0.075 M Pb(NO3)2. (Ksp PbCl2 = 1.7 x 10-5)
Will a ppt form?
Problems:
• Calculate the solubility of Al(OH)3(s) in 0.20 M Al(NO3)3:
• Calculate the solubility of PbI2(s) in 0.010 M NaI :
• Calculate the solubility of CaCO3 in water:
Drill: Calculate the solubility of PbS(s) in water:
(Ksp = 2.0 x 10-33)
(Ksp = 3.2 x 10-8)
( Ksp = 4.9 x 10-15)
( Ksp = 1.0 x 10-28)
Multiple Common Ion Problem Demonstration:
Problem: Aqueous NaI is added (dropwise) to a solution of 0.10 M AgNO3 & 0.20 M CuNO3 .
(AgI Ksp = 1.0 x 10-16)(CuI Ksp = 5.1 x 10-12) What substance precipitates 1st ?
A) Determine all ions present in solution:
B) Determine the concentration of each ion
NaI
 Na+ + I?
?
?
AgNO3  Ag+ + NO30.10 – all 0.10 0.10
CuNO3  Cu+ + NO30.20 – all 0.20 0.20
Perform equilibrium calculations with ions that appear in the Ksp’s that were given:
1)
2)
3)
4)
Set up & balance rxn:
Assign Eq. amounts:
Write the Keq’s:
Substitute:
5) Solve for x:
The solubility of
AgI < CuI
AgI  Ag+ + I0.10 – x x
x
+
Ksp1 = [Ag ][ I ]
Ksp1 = [0.10 + x][ x]
Ksp1 = 0.10 x
0.10 x = 1.0 x 10-16
x = 1.0 x 10-15 M
x = 1.0 x 10-15 M
>
CuI  Cu+ + I0.20 – y
y
y
Ksp2 = [0.20 + y][ y]
Ksp2 = [0.20 + y][ y] drop x & y
Ksp2 = 0.20 y
0.20 y = 5.1 x 10-12
y = 2.6 x 10-11 M
y = 2.6 x 10-11 M
Thus AgI ppts. 1st
Problem:
 Aqueous Kcl is added (dropwise) to a solution of 0.10 M AgNO3 & 0.20 M Cu(NO3)2 .
(AgCl Ksp = 1.8 x 10-10)
(CuCl2 Ksp = 2.4 x 10-16) What substance precipitates 1st ?
•
Calculate the solubility of PbCl2(s):
Drill: Calculate the solubility of Sc(OH)3(s):
(Ksp = 1.6 x 10-5)
(Ksp = 2.7 x 10-31)
Test Review
Problems:
• Calculate the solubility of AgI(s) :
(Ksp = 1.5 x 10-16)
• Calculate the solubility of AgI(s) in 0.20 M NaI :
(Ksp = 1.5 x 10-16)
• Calculate the solubility of MgZ2(s):
(Ksp = 4.0 x 10-15)
• The solubility of Ca(OH)2 = 4.0 x 10-6 M. Calculate its Ksp :
• Aqueous MgCl2 is added (dropwise) to a solution of 0.20 M KOH & 0.10 M K2SO4.
(Mg(OH)2 Ksp = 5.0 x 10-6) (MgSO4 Ksp = 5.6 x 10-5) What ppts. 1st ?
 10.0 mL 0.050 M KCl is added to 15 mL 0.060M Pb(NO3)2.(Ksp PbCl2 = 1.7 x 10-5) Will a ppt form?