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Exact Acid Base Treatment
Slide 1
Exact Treatment of Acid/Base Equilibria
Chem 6BH
Exact Acid Base Treatment
Slide 2
Exact Treatment: Preliminaries
Pure water has [H
3
O
+
] = [OH
–
] = 10
–7
and has been neglible when
compared to amounts generated from acids or bases present in
water.
– What if we have
very
weak acids or bases?
– What is we have
very
dilute solutions?
An EXACT treatment of acid base equilibria is needed!
Consider a weak acid HA with a concentration of
c
a
Consider a weak acid A
–
with a concentration of
c
b
In solution we will have the following:
H
3
O
+
, OH
–
, HA, A
–
, and M
+
(used to deliver A
–
)
Need 5 independent equations to relate c
a
, c
b
, and K
a
to concs. of
species present
Both in same
solution
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Exact Acid Base Treatment
Slide 3
Exact Treatment: Equations
Here are the equations we need to begin the exact solution:
Eqn A: [
M
+
]
=
c
b
(spectator ion with no effect)
Eqn B: [
H
3
O
+
][
OH
–
]
=
K
W
Eqn C:
[
H
3
O
+
][
A
–
]
[
HA
]
=
K
a
Eqn D:
c
a
+
c
b
=
[
HA
]
+
[
A
–
]
(conservation of mass/material)
Eqn E: [
M
+
]
+
[
H
3
O
+
]
=
[
OH
–
]
+
[
A
–
]
(charge balance)
NOTE: [ ] = conc at equilibrium
Exact Acid Base Treatment
Slide 4
Exact Treatment: The solution (1)
1. Get rid of M
+
:
c
a
+
c
b
=
[
HA
]
+
[
A
–
]=[
HA
]
+
c
b
+
[
H
3
O
+
]–[
OH
–
]
so
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This note was uploaded on 04/08/2008 for the course CHEM 6BH taught by Professor Hoeger during the Winter '08 term at UCSD.
 Winter '08
 HOEGER
 Bases

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