Chapter 17
Complex Aqueous Equilibria
Common Ion Effect
Buffers
Titrations
Predict using LeChatelier’s Principle
CH3COOH(aq)
 H+(aq) + CH3COO−(aq)
Consider the equilibrium involving the weak acid at 298K
shown above. What will happen to the pH if some solid
CH3COONa is added while the temperature is held
constant?
1. The p
pH will increase
2. The pH will decrease
3 The pH will not be effected
3.
effected.
4. There is not enough information to determine the effect
on the pH.
pH
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Test the prediction by doing an experiment.
DEMO:
CH3COOH(aq)

H+(aq) + CH3COO−(aq)
pH =
Add CH3COONa(s):
CH3COONa (aq) → Na+(aq) + CH3COO−(aq)
pH =
What happened to
[CH3COO−]?
Common Ion Effect: if a system
is at equilibrium and it is disturbed
by adding a strong electrolyte with
a common ion, the ionization of
the weak electrolyte will be
reduced.
d
d
[CH3COOH]?
[H+]?
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What happens when a strong acid is added to a
weak acid and to a mixture of the acid and its conjugate base?
CH3COOH(aq)
COOH( )  H+(aq)
( ) + CH3COO−(aq)
( )
pH
0 1M CH3COOH(aq)
0.1M
Add HCl
Mixture
0.1M CH3COOH
0.1M CH3COONa
pH
Add HCl
Add NaOH
Add NaOH
How does the addition of strong acid or base effect the pH
of the weak acid?
of the mixture?
Example of the
Common Ion effect
A Buffer is a solution that “resists” a change
g in p
pH
E.g.
E
g blood contains substances
that keep its pH fixed at 7.3 important for life functions
Buffer solutions consist of either:
http://chemcollective.org/activities/tutorials/buffers/buffers3
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How do Buffers work? When strong acid or base is added, the
ratio of weak acid: weak base does not change much.
If a small
amount of
hydroxide
y
is
added to an
equimolar
solution of HA
in NaA,, the HF
reacts with the
OH− to make
A− and water.
Buffer after
adding OH
Buffer with
[HA] = [A]
OH
[HA]
[A]
Buffer after
adding H+
Similarly, if
acid is added,
the A− reacts
with it to form
HA and water
water.
H+
[HA]
[A]
http://chemcollective.org/activities/tutorials/buffers/buffers3
[HA]
[A]
6
Quantitatively
C l l t th
Calculate
the pH
H off a solution
l ti th
thatt iis 0
0.1M
1M CH3COOH
and 0.1M CH3COONa
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We can derive an expression that will enable us to
calculate the p
pH of a buffer solution.
Consider the equilibrium constant expression for the
dissociation of a generic acid, HX:
HX(aq)
H+(aq) + X−(aq)
This is the Henderson–Hasselbalch
Henderson Hasselbalch equation.
equation
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Calculating the pH of a Buffer
What is the pH of a buffer that is 0.12 M in lactic acid,
CH3CH(OH)COOH, and 0.10 M in sodium lactate?
Ka for
f lactic
l ti acid
id iis 1
1.4
4 × 10−44.
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Calculating the pH of a Buffer
Calculate the pH of a solution that is 0.1M NH3 and 0.2M NH4Cl.
Kb for NH3 is 1.8 × 10−5.
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When Strong Acids or Bases Are Added
to a Buffer…
Buffer
reaction occurs.
occurs
Follow a 2-step process to determine pH
1. neutralization
2. pH determination
neutralization
A(aq) + H+(aq) → HA(aq)
Calculate
[HA] and [A]
Buffer
HA + A
pH
determination
neutralization
HA(aq) + OH(aq) → A(aq) + H2O()
Equilibrium
calculation
Stoichiometry calculation
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Finding the pH after adding
g Acid or Base to a Buffer
Strong
1. Determine how the neutralization reaction affects the amounts
of the weak acid and its conjugate base in solution.
2. Use the Henderson–Hasselbalch equation to determine the
new pH of the solution.
Add 0.020
0 020 mol OH
pH =???
Add 0.020
0 020 mol H+
Buffer:
0.300 M CH3COOH
0.300 M CH3COONa
pH =???
pH = 4.74
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Calculating pH changes in Buffers
A buffer is made by adding 0.300 mol HC2H3O2 and 0.300
mol NaC2H3O2 to enough water to make 1.00 L of solution.
The pH of the buffer is 4.74. Calculate the pH of this
solution after 0.020 mol of NaOH is added.
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Finding the pH after adding
g Acid or Base to a Buffer
Strong
1. Determine how the neutralization reaction affects the amounts
of the weak acid and its conjugate base in solution.
2. Use the Henderson–Hasselbalch equation to determine the
new pH of the solution.
Add 0.020
0 020 mol OH
Buffer:
0.280 M CH3COOH
0.320 M CH3COONa
pH = 4.80
4 80
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Add 0.020
0 020 mol H+
Buffer:
0.300 M CH3COOH
0.300 M CH3COONa
pH = 4.74
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Buffer:
0.320 M CH3COOH
0.280 M CH3COONa
pH = 4.68
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Choosing a Buffer
pH range of a buffer: range of pH over which the
buffer acts effectively
effectively.
optimal pH:
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Buffer Capacity: volume of acid or base that
can be added before the pH of the buffer
changes appreciably.
Whi h solution
Which
l ti will
ill have
h
the
th greatest
t t buffer
b ff capacity?
it ?
1M CH3COOH/CH3COONa
0.1M CH3COOH/CH3COONa
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Titration
A pH meter or indicators are used to
determine when the solution has reached
the equivalence point:
In this technique a known
concentration of base (or acid) is
slowly added to a solution of acid
(or base).
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The titration of a strong acid with a strong base can be
described byy a p
plot of p
pH vs. amount of base added.
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Quantitative determination of the pH for an acid
Acid Base Titration is a two step process
1. Neutralization
What is in solution after neutralization?
2. Find the pH
A) strong acid/base
B) weak/acid base
equilibrium calculation
buffer solution
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Sample calculation of pH during the titration
of a Strong Acid with a Strong Base
Titrate 0.1M HCl with 0.2M NaOH starting with 50 mL of HCl solution.
Calculate pH when the following quantities of NaOH are added:
a. 0 mL NaOH, b. 20 mL NaOH, c. 25 mL NaOH, d. 30 mL NaOH
a. Initial concentration of strong acid: (0mL of NaOH added)
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Sample calculation of pH during the titration
of a Strong Acid with a Strong Base
Titrate 0.1M HCl with 0.2M NaOH starting with 50 mL of HCl solution.
Calculate pH when the following quantities of NaOH are added:
a. 0 mL NaOH, b. 20 mL NaOH, c. 25 mL NaOH, d. 30 mL NaOH
b. concentration of solution after adding 20mL of NaOH:
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Sample calculation of pH during the titration
of a Strong Acid with a Strong Base
Titrate 0.1M HCl with 0.2M NaOH starting with 50 mL of HCl solution.
Calculate pH when 25 mL of NaOH is added.
Equivalence point is where moles of acid = moles of base
c. concentration of solution after adding 25mL of NaOH :
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Sample calculation of pH during the titration
of a Strong Acid with a Strong Base
Titrate 0.1M HCl with 0.2M NaOH starting with 50 mL of HCl solution.
Calculate pH when the following quantities of NaOH are added:
a. 0 mL NaOH, b. 20 mL NaOH, c. 25 mL NaOH, d. 30 mL NaOH
d. concentration of solution after adding 30mL of NaOH:
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The titration of a weak acid with a strong base can be
described byy a plot
p of p
pH vs. amount of base added.
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Calculating the pH during the Titration of a Weak Acid with a Strong Base is still a two step process.
g
pp
Sol’n with
HA and A
HA and A
neutralization
HA(aq) + OH
HA(aq) OH(aq) → A(aq) (aq) + H
H2O() Stoichiometry calculation
Determine [HA], [A]
[HA], [A
pH determination (buffer)
Equilibrium calculation
At each point below the equivalence point, the pH of the solution during titration is determined from the
the solution during titration is determined from the amounts of the acid and its conjugate base present at that particular time.
Buffer solutions are encountered during
titrations!
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Sample calculation of pH during the titration of a
Weak Acid with a Strong Base
Titrate 0.1M CH3COOH (Ka = 1.8 x 10−5) with 0.2M NaOH starting with 50 mL of
CH3COOH solution and calculate pH when the following quantities of NaOH are
added: a. 0 mL NaOH, b. 12.5 mL NaOH,
c. 25mL
d. 30 mL NaOH
a. Initial concentration of weak acid: (0mL of NaOH added)
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Sample calculation of pH during the titration of a
Weak Acid with a Strong Base
Titrate 0.1M CH3COOH (Ka = 1.8 x 10−5) with 0.2M NaOH starting with 50 mL of
CH3COOH solution and calculate pH when the following quantities of NaOH are
added: a. 0 mL NaOH, b. 12.5 mL NaOH,
c. 25mL
d. 30 mL NaOH
b. Half way to equivalence (12.5mL of NaOH added)
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Sample calculation of pH during the titration of a
Weak Acid with a Strong Base
Titrate 0.1M CH3COOH (Ka = 1.8 x 10−5) with 0.2M NaOH starting with 50 mL
of CH3COOH solution and calculate pH when the following quantities of NaOH
are added: a. 0 mL NaOH, b. 12.5 mL NaOH, c. 25 mL NaOH d. 30 mL
c. At the equivalence point (25mL of NaOH added)
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Sample calculation of pH during the titration of a
Weak Acid with a Strong Base
Titrate 0.1M CH3COOH (Ka = 1.8 x 10−5) with 0.2M NaOH starting with 50 mL
of CH3COOH solution and calculate pH when the following quantities of NaOH
are added: a. 0 mL NaOH, b. 12.5 mL,
c. 25mL
d. 30 mL NaOH
d. Beyond the equivalence point; excess strong base (30 mL of NaOH added)
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The initial pH of a weak acid and pH at equivalence after titration
with
ith a strong
t
base
b
d
depends
d on th
the value
l off Ka off the
th weak
k acid.
id
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INDICATORS are organic molecules that have
different colors for the acid and conjugate base.
InH+(aq)  In(aq) + H+(aq)
Some of the more common acid-base indicators
Name
pH interval for
Color change
Acid
color
Base
color
Methyl
y violet
1 - 2
Yellow
Violet
Methyl yellow
1.5 - 2.5
Red
Yellow
Methyl orange
2.5 - 4.0
Red
Yellow
Methyl red
4 2 - 6.3
4.2
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Red
Yellow
Bromthymol blue
6.0 - 7.6
Yellow
Blue
Thymol blue
8.0 - 9.6
Yellow
Blue
Phenolphthalein
8.5 - 10
Colorless Pink
Alizarin yellow G
10.1-12.0
Yellow
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Choose an indicator that will change in
the range of the equivalence point.
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Titration curve of a Weak Base with a Strong
Acid shows how p
pH changes
g with bases added.
Strong acid - strong base:
The pH at the equivalence
point
indicator of choice is:
Strong acid - weak base:
base:
The pH at the equivalence
point
indicator of choice is:
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Summary of expectations for pH at
q
in acid base titrations.
equivalence
Titrating a strong acid (base) with a strong base (acid)
pH at equivalence
Tit ti a weak
Titrating
k acid
id with
ith a strong
t
base
b
pH at equivalence
Titrating a weak base with a strong acid
pH at equivalence
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Titrations of Polyprotic
yp
Acids
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Find the pH of this solution?
A buffer solution is 0.20M in HPO42− and 0.10M in H2PO4−. What is the [H+]
in this solution?
H3PO4: Ka1 = 7.5  10−3
Ka2 = 6.2  10−8
12
Ka3 = 1  10−12
A.
B.
C.
D.
E.
3.7  10−3 M
3.7  10−4 M
3.1  10−8 M
3.1  10−9 M
5.0  10−13 M
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