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Lecture 19 3/14/05
Spring Break Quiz
Seminar today
Building a titration curve via calculations
Comparison of a titration of a strong acid vs. a weak acid
Region x-axis (mL of acid/base)
0.1 M NaOH
y-axis (pH)
100 mL of 0.1 M HCl
y-axis (pH)
100 mL of 0.1 M HF
Initial pH 0 1
Before the equivalence point(1/2 equiv. point)
50 1.47
equivalence point 100 7
After the equivalence point
150 12.3
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200
mL 0.1 M NaOH
pH
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200
mL 0.1 M NaOH
pH
Initial pH (NaOH titrating HF)
Ka = 6.8 x 10-4
HF ↔ F- + H+
I 0.1 0 0
C -X +X +X
E 0.1 - X X X
0108.6X108.6X
XX108.6108.6
)X1.0(
)X)(X(108.6
]HF[
]F][H[K
542
255
4
a
10.2pH
1091.7X
)1(2
)108.6)(1(4)108.6(108.6X
a2
ac4bbX
3
5244
2
Building a titration curve via calculations
Comparison of a titration of a strong acid vs. a weak acid
Region
x-axis (mL of acid/base)
0.1 M NaOH
y-axis (pH)
100 mL of 0.1 M HCl
y-axis (pH)
100 mL of 0.1 M HF
Initial pH 0 1 2.10
Before the equivalence point(1/2 equiv. point)
50 1.47
equivalence point 100 7
After the equivalence point
150 12.3
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200
mL 0.1 M NaOH
pH
Equivalence point (NaOH titrating HF)
X-axismoles OH- added = moles H3O+ in solution
COHVOH = CHVH
(0.1 M)VOH = (0.1 M)(100 mL)
VOH = 100 mL
Y-axisNeutralization of weak acid by strong basepH > 7
Initial pH (NaOH titrating HF)
Ka = 6.8 x 10-4
H2O + F- ↔ HF + OH-
I 0.05 0 0
C -X +X +X
E 0.05 - X X X
7
211
11
b
1057.8X
)05.0(
)X(1047.1
)X05.0(
)X)(X(1047.1
]F[
]HF][OH[K
HF + OH- ↔ F- + H2O
0.01 moles 0.01 moles 0 moles
0 moles 0.moles 0.01 moles
pOH = 6.07
pH = 7.93
Building a titration curve via calculations
Comparison of a titration of a strong acid vs. a weak acid
Region
x-axis (mL of acid/base)
0.1 M NaOH
y-axis (pH)
100 mL of 0.1 M HCl
y-axis (pH)
100 mL of 0.1 M HF
Initial pH 0 1 2.10
Before the equivalence point(1/2 equiv. point)
50 1.47
equivalence point 100 7 7.93
After the equivalence point
150 12.3
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200
mL 0.1 M NaOH
pH
½ Equivalence point (NaOH titrating HF)
X-axismoles OH- added = ½ moles H3O+ in solution
Vequivalence = 100 mL, so V1/2equivalence = 50 mL
Y-axis HF + OH- F- + H2O(0.1 L)(0.1 M) (0.05 L)(0.1 M)0.01 moles 0.005 moles 0 moles0.005 moles 0 moles 0.005 moles
16.3pH005.0
005.0log16.3pH
]HF[
]F[logpKpH a
pH = pKa
Always true for titrations of either weak acids or weak bases
Building a titration curve via calculations
Comparison of a titration of a strong acid vs. a weak acid
Region
x-axis (mL of acid/base)
0.1 M NaOH
y-axis (pH)
100 mL of 0.1 M HCl
y-axis (pH)
100 mL of 0.1 M HF
Initial pH 0 1 2.10
Before the equivalence point(1/2 equiv. point)
50 1.47 3.16
equivalence point 100 7 7.93
After the equivalence point
150 12.3
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200
mL 0.1 M NaOH
pH
After Equivalence point (NaOH titrating HF)
X-axisCan pick any volume greater than Vequivalence
Often pick 3/2 equivalence volume = 150 mL
Y-axis HF + OH- F- + H2O(0.1 L)(0.1 M) (0.15 L)(0.1 M)0.01 moles 0.015 moles 00 moles 0.005 moles 0.01 moles
pH is dominated by excess OH-
Total volume = volume of acid solution + volume of base added = 250 mL
pOH = -log (0.005 moles/0.25 L) = 1.7pH = 14 – 1.7 = 12.3
Building a titration curve via calculations
Comparison of a titration of a strong acid vs. a weak acid
Region
x-axis (mL of acid/base)
0.1 M NaOH
y-axis (pH)
100 mL of 0.1 M HCl
y-axis (pH)
100 mL of 0.1 M HF
Initial pH 0 1 2.10
Before the equivalence point(1/2 equiv. point)
50 1.47 3.16
equivalence point 100 7 7.93
After the equivalence point
150 12.3 12.3
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200
mL 0.1 M NaOH
pH
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200
mL 0.1 M NaOH
pH
