Weak base and polyprotic acid titrations
Announcements
Monday, April 19, 2010
MasteringChemistry due dates:
Ch 16: Tuesday Apr 20, 11:59pm
Ch 17: Tuesday Apr 27, 11:59pm
Ch 18: Friday May 7, 11:59pm
pH
Quiz 3 Wednesday Apr 21 on end of Ch 15 and Ch 16.
Vol HCl added (mL)
Polyprotic acid titr by strong base
pH
Vol NaOH added (mL)
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Solubility equilibria
pH Indicators
Compounds we called "insoluble" before, are actually
very slightly soluble, governed by an equilibrium:
CaF2(s) ⇌ Ca2+(aq) + 2 F−(aq)
Solubility product constant, Ksp for CaF2 =
Molar solubility, S =
Make sure the color range of your indicator is within the
most vertical part of the titration curve!
mol solute (when saturated)
L solution
If the molar solubility, S, of PbF2 is 2.61 x 10−3 M, what is
the Ksp of PbF2?
PbF2(s) ⇌
What are the ion concentrations?
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Calculating molar solubility
Ksp and relative solubility
If Ksp for Mg(OH)2 is 2.06 x 10−13, calculate the molar
solubility, S.
Direct comparison of Ksp values only works between
compounds that dissociate into the same number of
ions.
Which is the most soluble?
BaF2
CaF2
Fe(OH)2
PbCl2
PbBr2
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Ksp = 2.45 x 10−5
Ksp = 1.46 x 10−10
Ksp = 4.87 x 10−17
Ksp = 1.17 x 10−15
Ksp = 4.67 x 10−6
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Common Ion Effect
Common ion effect calculation
How is solubility affected when more ions are added to
the solution?
What is S for Ca(OH)2? Ksp = 4.68 x 10−6
Ca(OH)2(s) ⇌ Ca2+(aq) + 2 OH−(aq); Ksp = 4.68 x 10−6
To this solution, we add Ca(NO3)2
Na+, K+, NH4+,
NO3−, C2H3O2−
make compounds
totally soluble
The reaction shifts:
What is S for Ca(OH)2 in 0.10 M Ca(NO3)2 solution?
A common ion in solution will _________ a compound's
solubility.
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Effect of pH on solubility
Precipitation
If a compound's dissociation produces a basic anion
(usually OH−, CO32−, S2−), solubility is dependent on pH.
To calculate whether or not a precipitate will form, take
the given ion concentrations and calculate Q (the
reaction quotient).
CaCO3(s) ⇌ Ca2+(aq) + CO32−(aq)
If Q = Ksp:
Addition of strong acid to this solution…
If Q > Ksp:
If Q < Ksp:
A similar effect occurs when base is added to a solution
which contains an acidic cation.
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A newly mixed solution has these ion concentrations:
[Pb2+] = 0.25 M, [Cl−] = 0.017 M. Will a precipitate form?
Ksp (PbCl2) = 1.17 x 10−5.
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