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8/13/2019 Chapter20 Solubility Product Constant
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20Ionic Equilibria III:
The SolubilityProduct Principle
8/13/2019 Chapter20 Solubility Product Constant
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Chapter Goals
1. Solubility Product Constants
2. Determination of Solubility ProductConstants
3. Uses of Solubility Product Constants
4. Fractional Precipitation
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Solubility Product Constants
• Silver chloride, AgCl,is rather insoluble in water.
• Careful experiments show that if solid AgCl is
placed in pure water and vigorously stirred, a
small amount of the AgCl dissolves in the water.
aqaq ClAgClAg
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Solubility Product Constants
• The equilibrium constant expression for this
dissolution is called a solubility product constant.
– Ksp = solubility product constant
-10-
sp 101.8]][Cl[AgK
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Solubility Product Constants
• The solubility product constant, Ksp, for a
compound is the product of the concentrations of
the constituent ions, each raised to the power
that corresponds to the number of ions in oneformula unit of the compound.
• Consider the dissolution of silver sulfide in water.
Ag S 2 Ag + S2+ 2- H O + 2-2
100%
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Solubility Product Constants
• The solubility product expression for Ag2S is:
K Ag Ssp2 2
10 10 49.
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Solubility Product Constants
• The dissolution of solid calcium phosphate in
water is represented as:
22+ 3 2 3
3 4 ( ) 4 ( )2 s100Ca PO 3 Ca 2 POo
H O
aq aqC
The solubility product constant expression is:
You do it!
K Ca POsp2 3
43 2
1 0 10 25
.
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Solubility Product Constants
• In general, the dissolution of a slightly solublecompound and its solubility product expressionare represented as:
2s r
r s s100
r ss r
sp
M Y r M s Y
K M Y
o
H O
C
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Solubility Product Constants
• The same rules apply for compounds that have
more than two kinds of ions.
• One example of a compound that has more than
two kinds of ions is calcium ammoniumphosphate.
3
44
2
sp
3
aq4
1
aq4
2
aqs44
PO NHCaK
PO NHCaPOCaNH
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Determination of Solubility
Product Constants• The equation for the dissociation of silver
chloride, the appropriate molar concentrations,
and the solubility product expression are:
AgCl Ag Cl
1.34 10 1.34 10K Ag Cl
s
-5 -5
sp
M M
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Determination of Solubility
Product Constants• Substitution of the molar concentrations into the
solubility product expression gives:
K Ag Clsp
134 10 134 10
18 10
5 5
10
. .
.
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Determination of Solubility
Product ConstantsExample 20-2: One liter of saturated calcium
fluoride solution contains 0.0167 gram of CaF2 at
25oC. Calculate the molar solubility of, and Ksp for,
CaF2.
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Uses of Solubility
Product Constants• The solubility product constant can be used to
calculate the solubility of a compound at 25oC.
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Uses of Solubility
Product ConstantsExample 20-3: Calculate the molar solubility of
barium sulfate, BaSO4, in pure water and the
concentration of barium and sulfate ions in
saturated barium sulfate at 25o
C. For bariumsulfate, Ksp= 1.1 x 10-10.
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Uses of Solubility
Product ConstantsExample 20-4: The solubility product constant for
magnesium hydroxide, Mg(OH)2, is 1.5 x 10-11.
Calculate the molar solubility of magnesium
hydroxide and the pH of a saturated magnesiumhydroxide solution at 25oC.
You do i t !
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The Common Ion Effect in
Solubility CalculationsExample 20-5: Calculate the molar solubility of
barium sulfate, BaSO4, in 0.010 M sodium sulfate,
Na2SO4, solution at 25oC. Compare this to the
solubility of BaSO4 in pure water. (Example 20-3).(What is the common ion?)
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The Common Ion Effect in
Solubility Calculations
• The molar solubility of BaSO4 in 0.010 M Na2SO4
solution is 1.1 x 10-8 M .
• The molar solubility of BaSO4 in pure water is 1.0 x
10-5 M .
– BaSO4 is 900 times more soluble in pure water than in
0.010 M sodium sulfate!
– Adding sodium sulfate to a solution is a fantastic
method to remove Ba2+ ions from solution!
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The Reaction Quotient in
Precipitation Reactions• The reaction quotient, Q, and the Ksp of a
compound are used to calculate the
concentration of ions in a solution and
whether or not a precipitate will form.
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The Reaction Quotient in
Precipitation Reactions
Qsp = Ksp saturated solution
Qsp < Ksp unsaturated sol’n; precipitation cannot occur
Qsp > Ksp supersaturated sol’n; precipitation should occur
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The Reaction Quotient in
Precipitation ReactionsExample 20-6: We mix 100 mL of 0.010 M
potassium sulfate, K2SO4, and 100 mL of 0.10 M
lead (II) nitrate, Pb(NO3)2 solutions. Will a
precipitate form?
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The Reaction Quotient in
Precipitation ReactionsExample 20-7: Suppose we wish to remove
mercury from an aqueous solution that contains a
soluble mercury compound such as Hg(NO3)2. We
can do this by precipitating mercury (II) ions as theinsoluble compound HgS. What concentration of
sulfide ions, from a soluble compound such as
Na2S, is required to reduce the Hg2+ concentration
to 1.0 x 10-8 M ? For HgS, Ksp=3.0 x 10-53.You do i t !
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The Reaction Quotient in
Precipitation ReactionsExample 20-7: What concentration of sulfide ions,
from a soluble compound such as Na2S, is required
to reduce the Hg2+ concentration to 1.0 x 10-8 M ?
For HgS, Ksp=3.0 x 10-53
.
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The Reaction Quotient in
Precipitation ReactionsExample 20-8: Refer to example 20-7. What volume
of the solution (1.0 x 10-8 M Hg2+ ) contains 1.0 g of
mercury?
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Fractional Precipitation
• The method of precipitating some ions from a
solution while leaving others in solution is called
fractional precipitation.
– If a solution contains Cu+
, Ag+
, and Au+
, each ion canbe precipitated as chlorides.
13
sp
10sp
7
sp
100.2ClAuK ClAuAuCl
108.1ClAgK ClAgAgCl
109.1ClCuK ClCuCuCl
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Fractional Precipitation
Example 20-9: If solid sodium chloride is slowly
added to a solution that is 0.010 M each in Cu+, Ag+,
and Au+ ions, which compound precipitates first?
Calculate the concentration of Cl-
required to initiateprecipitation of each of these metal chlorides.
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Fractional Precipitation
• These three calculations give the [Cl-] required
to precipitate AuCl ([Cl-] >2.0 x 10-11 M ), to
precipitate AgCl ([Cl-] >1.8 x 10-8 M ), and to
precipitate CuCl ([Cl-] >1.9 x 10-5 M ).• It is also possible to calculate the amount of
Au+ precipitated before the Ag+ begins to
precipitate, as well as the amounts of Au+ and
Ag+ precipitated before the Cu+ begins to
precipitate.
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Fractional Precipitation
Example 20-10: Calculate the percentage of Au+
ions that precipitate before AgCl begins to
precipitate.
– Use the [Cl-
] from Example 20-9 to determine the [Au+
]remaining in solution just before AgCl begins to
precipitate.
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Fractional Precipitation
• The percent of Au+ ions unprecipitated just
before AgCl precipitates is:
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Fractional Precipitation
• A similar calculation for the concentration
of Ag+ ions unprecipitated before CuCl
begins to precipitate is:
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Fractional Precipitation
• The percent of Ag+ ions unprecipitated justbefore AgCl precipitates is:
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Ionic Equilibria III:
The SolubilityProduct Principle
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