1
Chem215/P.Li/Monoprotic AcidBase Equilibria/P 1
Monoprotic acidbase equilibria
(Ch 10)
•
Weak acid equilibria
•
Fraction of dissociation of a weak acid
•
Weak base equilibria
•
Fraction of association of a weak base
•
When the dissociation of water cannot be neglected
•
Buffer, HendersonHasselbalch equation and buffer capacity
Chem215/P.Li/Monoprotic AcidBase Equilibria/P 2
Weak acid equilibria
To calculate the pH of 0.050F of
o
hydroxybenzoic acid (K
a
=1.07
×
10
3
)
OH
COOH
OH
COO

+
H
+
Initial conc. (M)
0.050
0
0
Final conc. (M)
0.050x
x
x
We assume that there is negligible contribution of H
+
from dissociation of water.
]
[
]
][
[
HA
A
H
K
Since
a

+
=
x
x

=
×

050
.
0
10
07
.
1
2
3
0
10
35
.
5
)
10
07
.
1
(
5
3
2
=
×

×
+


x
x
This is a quadratic equation: ax
2
+bx+c=0
a
ac
b
b
x
2
4
2

±

=
∴
)
1
(
2
)
10
35
.
5
)(
1
(
4
)
10
07
.
1
(
10
07
.
1
5
2
3
3



×


×
+
×

=
The negative root is rejected
.
3
10
80
.
6

×
=
Chem215/P.Li/Monoprotic AcidBase Equilibria/P 3
Weak acid equilibria
The amount of H
+
, which is similar to [OH

], contributed from dissociation of
water is very small and the above approximation is justified.
]
[
]
[
+

=
H
K
OH
Since
W
3
14
10
80
.
6
10
0
.
1


×
×
=
M
12
7
10
4
.
1

×
=
x
H
pH
log
]
log[

=

=
∴
+
17
.
2
=
If the concentration of the acid is larger (e.g. 5.0F) and/or Ka is
smaller, another approximation: 5.0x
≈
5.0 could also be made to
avoid solving the quadratic equation.
Chem215/P.Li/Monoprotic AcidBase Equilibria/P 4
Fraction of dissociation of a weak acid
acid
the
of
ion
concentrat
formal
base
conjugate
d
dissociate
of
amount
=
a
F
x
HA
A
A
=
+
=


]
[
]
[
]
[
So 13.6% of the 0.050F acid has dissociated to give H
+
, leading to a rise in [H
+
],
or a decrease in pH.
M
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 Spring '10
 aa
 pH, cocaine, buffer solution, Chem215/P.Li/Monoprotic AcidBase Equilibria/P

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