17B. An Acid–Base Titration Curve
Time: 2 hours
Required chemicals and solutions:
Reagent
Requirement/5 Pairs
Preparation of 1 L
Potassium hydrogen phthalate
NaOH, 0.1 M
5.1 g
250 mL
—
4.0 g of NaOH
Other required materials:
Balance, precise (at least 0.001 g), 18 students
pH meter with electrode and pH 5.00 buffer, 12 pairs
Buret, 50 mL, 1pair
Kimwipes
Special note:
1. The time requirement is valid only if each student works with a partner and if there is
at least one pH meter for every two pairs of students.
Connections: Sodium hydroxide is also used to manufacture soap.
196
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An Acid–Base Titration Curve
Date:
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Student name:
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Course:
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Team members:
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Section:
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Instructor:
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Prelaboratory assignment
1. Provide definitions for the following terms:
a. Acid–base titration curve
A plot of the pH of a solution of acid (or base)
against the volume of added base (or acid)
b. Equivalence point
The point at which a stoichiometric amount of a
reactant has been added during a titration
c.
Standardization
The procedure by which we obtain the molarity of a
solution of one substance from an accurately known
amount of another substance
d. pH
pH log[H3O]
e. pH meter
A meter that accurately measures the pH of a
solution
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197
198 Experiment 17B
2. Estimate the mass of KHP that will require 25 mL of 0.10 M NaOH to
reach the equivalence point in a titration. Give the chemical equation for
the reaction.
KHC8H4O4(aq) NaOH(aq) l NaKC8H4O42(aq) H2O(l)
Molar mass of KHP 204.2244 g/mol
Mass KHP 204.2244 g KHP
1 mol KHP
0.025 L
1 mol KHP
1 mol NaOH
0.10 mol NaOH/L
0.51 g
3. Calculate the molarity of the acetic acid solution in Figure 17B.1 if
25.0 mL of that solution has been titrated with the 0.101 M solution
of NaOH.
HC2H3O2(aq) NaOH(aq) l NaC2H3O2(aq) H2O(l)
0.02702 L 0.101 mol NaOH/L Molarity HC2H3O2 soln. 0.0250 L
1 mol HC2H3O2
1 mol NaOH
0.109 M
Copyright © Houghton Mifflin Company. All rights reserved.
An Acid–Base Titration Curve
Date:
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Student name:
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Course:
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Team members:
..................................................................................................
Section:
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..................................................................................................
Instructor:
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Results
Sample
1
2
Mass of KHP and paper (g)
..........................................
0.8626
..........................................
Mass of paper (g)
..........................................
0.4036
..........................................
Mass of KHP (g)
..........................................
0.4590
..........................................
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0.8991
0.4311
0.4680
199
200 Experiment 17B
Sample 1
Initial pH:
3.42
............................
Initial buret reading (mL):
Buret
Reading (mL)
Volume
Added (mL)
pH
2.28
............................
............................
3.36
............................
2.12
............................
4.38
............................
3.14
............................
5.20
............................
3.96
............................
6.52
............................
5.28
............................
7.27
............................
6.03
8.30
............................
7.06
............................
9.35
............................
8.11
............................
10.32
............................
9.08
............................
11.37
............................
10.13
............................
............................
12.26
............................
13.22
14.34
............................
15.26
............................
16.46
............................
17.27
............................
............................
18.35
............................
............................
19.22
20.44
21.28
............................
............................
............................
............................
............................
............................
............................
............................
............................
............................
............................
............................
............................
............................
1.04
1.24
............................
Buret
Reading (mL)
Volume
Added (mL)
pH
3.76
............................
22.34
............................
21.10
..........................
4.10
............................
23.30
............................
22.06
..........................
4.31
............................
24.26
............................
23.02
..........................
4.37
............................
25.26
............................
24.02
..........................
4.57
............................
25.44
............................
24.20
..........................
............................
4.61
............................
25.66
............................
24.42
..........................
............................
4.68
............................
25.84
............................
24.60
..........................
4.80
............................
26.05
............................
24.81
..........................
4.90
............................
26.26
............................
25.02
..........................
4.96
............................
27.36
............................
26.12
..........................
5.02
............................
28.25
............................
27.01
..........................
5.09
............................
29.37
............................
28.13
..........................
5.15
............................
30.74
............................
29.50
..........................
5.22
............................
31.72
............................
30.98
..........................
5.32
............................
............................
..........................
5.36
............................
............................
..........................
5.46
............................
............................
..........................
11.02
............................
............................
11.98
............................
............................
13.10
............................
............................
14.02
............................
............................
15.22
............................
16.03
............................
5.92
6.10
6.33
6.91
7.89
8.68
10.51
10.79
11.19
11.32
11.49
11.68
11.74
11.84
17.11
............................
............................
17.98
............................
5.51
............................
............................
..........................
............................
19.20
............................
5.68
............................
............................
..........................
20.04
............................
5.80
............................
............................
..........................
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An Acid–Base Titration Curve 201
Sample 2
Initial pH:
3.50
............................
Initial buret reading (mL):
Buret
Reading (mL)
Volume
Added (mL)
1.24
............................
2.36
............................
3.65
4.73
............................
............................
5.85
............................
7.00
............................
8.48
............................
............................
9.69
............................
10.80
............................
............................
12.00
............................
13.00
14.34
............................
15.69
16.85
............................
17.90
............................
19.33
20.40
21.54
............................
............................
............................
............................
............................
............................
............................
............................
............................
............................
............................
............................
0.02
............................
pH
1.22
............................
2.34
............................
............................
3.63
............................
4.71
............................
5.83
............................
6.98
............................
8.46
............................
............................
9.67
............................
10.78
............................
Buret
Reading (mL)
Volume
Added (mL)
pH
3.82
............................
23.08
............................
23.06
..........................
6.22
4.14
............................
24.24
............................
24.22
..........................
4.40
............................
24.42
............................
24.40
..........................
4.48
............................
24.63
............................
24.61
..........................
4.58
............................
24.88
............................
24.86
..........................
4.73
............................
25.06
............................
25.04
..........................
4.85
............................
25.30
............................
25.28
..........................
4.90
............................
26.34
............................
26.32
..........................
4.99
............................
27.41
............................
27.39
..........................
5.07
............................
28.63
............................
28.61
..........................
5.14
............................
29.73
............................
29.71
..........................
5.25
............................
30.62
............................
30.60
..........................
5.31
............................
31.64
............................
31.62
..........................
5.40
............................
............................
..........................
6.72
6.90
7.17
9.32
10.72
10.91
11.29
11.48
11.98
............................
............................
12.98
............................
............................
14.32
............................
............................
15.67
............................
............................
16.83
............................
............................
17.88
............................
5.50
............................
............................
..........................
............................
19.31
............................
5.68
............................
............................
..........................
............................
20.38
............................
5.74
............................
............................
..........................
21.52
............................
5.96
............................
............................
..........................
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11.66
11.76
11.84
11.90
202 Experiment 17B
Questions
1. a. Use the graph paper that is available to plot the titration curves.
Locate the equivalence point on each graph. Complete the following
table.
Sample
1
2
24.40
..........................................
8.60
..........................................
Volume at equivalence point (mL)
..........................................
pH at equivalence point
..........................................
24.80
8.70
b. Calculate the molarity of the NaOH solution from each result, and
calculate the mean.
Sample 1:
0.4590 g KHP 204.2244 g KHP 1L
24.40 mL 10 mL
1 mol KHP
1 mol NaOH
1 mol KHP
3
0.09211 M
Sample 2:
0.4680 g KHP 204.2244 g KHP 24.80 mL 101 LmL
1 mol KHP
3
1 mol NaOH
1 mol KHP
0.09240 M
Mean value 0.0923 M
The precision justifies only three significant figures.
c. Use your graphs to obtain the data required in the following table.
Sample
1
2
Volume at halfway point (mL)
..........................................
12.20
..........................................
pH at halfway point
..........................................
5.10
..........................................
pKa
..........................................
5.10
..........................................
Mean pKa
12.40
5.10
5.10
5.10
..........................................
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An Acid–Base Titration Curve 203
d. Use the mean pKa to calculate Ka for KHP.
Ka antilog(5.10) 7.9 106
2. Use your value of Ka to calculate the initial pH for each of your samples
(Ebbing/Gammon, Section 17.1), and compare the calculated and
experimental results.
Sample 1: obs. pH 3.42
Mol KHP 2.248 103 mol
6
7.9 10
x2
2.248 103 mol/0.050 L
x [H3O] 6.0 104 M
pH 3.22
Sample 2: obs. pH 3.50
Mol KHP 2.292 103 mol
6
7.9 10
x2
2.292 103 mol/0.050 L
x [H3O] 6.0 104 M
pH 3.22
3. a. Calculate Kb for the C8H4O42 anion from the value of Ka that you
obtained for KHP.
Kb Kw /Ka
1.00 1014/7.9 106 1.3 109
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204 Experiment 17B
b. Use Kb, the number of moles of C8H4O42 at the equivalence point,
and the total volume at that point to calculate the pH for each
sample at the equivalence point. Compare these calculated results
with the experimental results.
Sample 1: obs. pH 8.60
9
1.3 10
x2
2.248 103 mol/0.074 L
x [OH] 6.3 106 M
pOH 5.20
pH 8.80
Sample 2: obs. pH 8.70
9
1.3 10
x2
2.292 103 mol/0.074 L
x [OH] 6.3 106 M
pOH 5.20
pH 8.80
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