Solubility and ComplexIon Equilibria

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Solubility and Complex-Ion Equilibria

• Chapter 17

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Solubility Equilibria
Consider the reaction shown below Ca2 (aq)
2 F- CaF2 (s) What is the equilibrium
constant expression?

• K 1/Ca2F-2
• Ksp Ca2F-2 The Solubility Product
• A constant
• The solubility is the concentration of solid
  dissolved at equilibrium.

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An Example

• Determine the Ksp of Bi2S3 if the solubility is
  1.0 x 10-15 M.

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An Example

• Determine the Ksp of Bi2S3 if the solubility is
  1.0 x 10-15 M.

Bi2S3 (s) 2 Bi3 (aq) 3 S2- (aq)
KSP Bi32S2-3 ?
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An Example

• Determine the Ksp of Bi2S3 if the solubility is
  1.0 x 10-15 M.

Bi2S3 (s) 2 Bi3 (aq) 3 S2- (aq)
KSP Bi32S2-3 ?
Solubility is the concentration of solid
dissolved at equilibrium.
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An Example

• Determine the Ksp of Bi2S3 if the solubility is
  1.0 x 10-15 M.

Bi2S3 (s) 2 Bi3 (aq) 3 S2- (aq)
X
2X
3X
KSP Bi32S2-3 ?
Solubility is the concentration of solid
dissolved at equilibrium.
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An Example

• Determine the KSP of Bi2S3 if the solubility is
  1.0 x 10-15 M.

Bi2S3 (s) 2 Bi3 (aq) 3 S2- (aq)
X
2X
3X
KSP Bi32S2-3 ?
Solubility is the concentration of solid
dissolved at equilibrium.
2(1.0 x 10-15)23(1.0 x 10-15)3
1.1 x 10-73
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A Question

• Which of the following salts is the least
  soluble?
• AgCl Ksp 1.6 x 10-10
• AgI Ksp 1.5 x 10-16
• AgBr Ksp 5.0 x 10-13
• Mg(OH)2 Ksp 8.9 x 10-12
• Ag3PO4 Ksp 1.8 x 10-18

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The Solution
AgCl (s) Ag (aq) Cl- (aq)

• Ksp 1.6 x 10-10 AgCl- X2
• X 1.3 x 10-5

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A Question

• Which of the following salts is the most soluble?
• AgCl Ksp 1.6 x 10-10 X 1.3 x 10-5 M
• AgI Ksp 1.5 x 10-16
• AgBr Ksp 5.0 x 10-13
• Mg(OH)2 Ksp 8.9 x 10-12
• Ag3PO4 Ksp 1.8 x 10-18

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A Question

• Which of the following salts is the most soluble?
• AgCl Ksp 1.6 x 10-10 X 1.3 x 10-5 M
• AgI Ksp 1.5 x 10-16 X 1.2 x 10-8 M
• AgBr Ksp 5.0 x 10-13 X 7.1 x 10-7 M
• Mg(OH)2 Ksp 8.9 x 10-12 X 1.3 x 10-4 M
• Ag3PO4 Ksp 1.8 x 10-18 X 1.6 x 10-5 M

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Types of Calculations Involving Ksp

• Determine Ksp
• Need a concentration, mass/volume, pressure
• Determine solubility
• Need Ksp
• Determine concentration at different pHs
• Common-ion effects

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A Question of KSP

• Calculate the solubility product constant for
  copper(II) iodate, Cu(IO3)2. The solubility of
  copper(II) iodate in water is 0.13 g/100 mL.

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A Question of KSP

• Calculate the solubility product constant for
  copper(II) iodate, Cu(IO3)2. The solubility of
  copper(II) iodate in water is 0.13 g/100 ml.

Calculate the molar solubility first,
0.13 g L
1 mol 413 g
x
3.15 x 10-3 M
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A Question of KSP

• Calculate the solubility product constant for
  copper(II) iodate, Cu(IO3)2. The solubility of
  copper(II) iodate in water is 0.13 g/100 ml.

0.13 g L
1 mol 413 g
x
3.15 x 10-3 M
Then construct the concentration table,
Cu(IO3)2 (s) Cu2 2 IO3- Starting
0 0 Change 0.00315
0.00315 Equilibrium 0.00315 2(0.00315)
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A question of Ksp

• Calculate the solubility product constant for
  copper(II) iodate, Cu(IO3)2. The solubility of
  copper(II) iodate in water is 0.13 g/100 ml.

0.13 g L
1 mol 413 g
x
3.15 x 10-3 M
Cu(IO3)2 (s) Cu2 2 IO3- Starting
0 0 Change 0.00315
0.00315 Equilibrium 0.00315 2(0.00315)
And solve the solubility equilibrium expression,
Ksp Cu2IO3-2 (0.00315)2(0.00315)2
1.3 x 10-7
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Common-Ion Effect

• What is the solubility of strontium sulfate,
  SrSO4, in 0.15 M sodium sulfate, Na2SO4? Ksp
  (SrSO4) 2.5 x 10-7

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Common Ion Effect

• What is the solubility of strontium sulfate,
  SrSO4, in 0.15 M sodium sulfate, Na2SO4? Ksp
  (SrSO4) 2.5 x 10-7

The initial concentration of SO42- is 0.15 M,
SrSO4 (s) Sr2 SO42- Starting 0
0.15 Change x
x Equilibrium x x 0.15
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Common Ion Effect

• What is the solubility of strontium sulfate,
  SrSO4, in 0.15 M sodium sulfate, Na2SO4? Ksp
  (SrSO4) 2.5 x 10-7

SrSO4 (s) Sr2 SO42- Starting 0
0.15 Change x
x Equilibrium x x 0.15
Solve solubility product expression for x,
Sr2SO42- Ksp (x)(0.15 x) ? (x)(0.15) ?
2.5 x 10-7 x 1.7 x 10-6 M
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Common Ion Effect

• What is the solubility of strontium sulfate,
  SrSO4, in 0.15 M sodium sulfate, Na2SO4? KSP
  (SrSO4) 2.5 x 10-7

SrSO4 (s) Sr2 SO42- Starting 0
0.15 Change x
x Equilibrium x x 0.15
Sr2SO42- Ksp (x)(0.15 x) ? (x)(0.15) ?
2.5 x 10-7 x 1.7 x 10-6 M
And convert to g/L
184 g 1 mol
1.7 x 10-6 mol L
x
3.1 x 10-4 g/L
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A Question of pH

• Calculate the concentration of Pb2 in each of
  the following solutions and determine which pH is
  better for removing lead from drinking water.
• A saturated solution of Pb(OH)2 Ksp 1.2 x
  10-15
• In a) pH 7.00 or b) pH 13.00
• Pb(OH)2 (s) Pb2 (aq) 2OH- (aq)

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A Ksp Problem Solved

• Since pH is describing both H and OH-, we
  can first determine OH-
• pH 7.00 14.00 - pOH or pOH
  7.00

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A Ksp Problem Solved

• Since pH is describing both H and OH-, we
  can first determine OH-
• pH 7.00 14.00 - pOH or pOH
  7.00 So OH- 1.0 x 10-7 M

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A Ksp Problem Solved pH 7

• Since pH is describing both H and OH-, we
  can first determine OH-
• pH 7.00 14.00 - pOH or pOH
  7.00 So OH- 1.0 x 10-7 M
• Ksp 1.2 x 10-15 Pb21 x 10-72
• Pb2 0.12 M

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A Ksp Problem Solved pH 13

• Since pH is describing both H and OH-, we
  can first determine OH-
• pH 13.00 14.00 - pOH or pOH
  1.00 So OH- 0.10 M
• Ksp 1.2 x 10-15 Pb20.12
• Pb2 1.2 x 10-13 M

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Therefore,

• Thus, Pb2 0.10 M at pH 7.00 1.0 x
  10-13 M at pH 13.00
• At higher pH, the solubility of Pb2 decreases.