Chemistry Reference
In-Depth Information
a
2
c
2
K
a
------
(1
a
)
c
a
2
c
K
a
------
(1
a
)
For weak electrolytes,
a
is very small and may be neglected so (1
a
) is
approximately
1. The simplified expression may now be written as
K
a
a
2
c
where
c
is the concentration, in moles per litre, and
a
is the degree of
ionisation of the acid. Then
(
—
)
K
a
a
c
The pH of the solution can now be determined:
[H
]
ac
Therefore,
(
—
)
(
K
a
c
)
K
a
[H
]
c
c
Taking logarithms,
5
log
K
a
5
log[H
]
log
c
Multiplying throughout by
1 gives
4
log
K
a
4
log[H
]
log
c
Therefore,
5
p
K
a
5
pH
log c
(1.2)
Equation (1.2) applies to the ionisation of weak acids, but a similar expres-
sion can be derived for weak bases. The equation for the ionisation of a
weak base may be expressed as
B
H
2
O
3
BH
OH
(1
a
)
c
ac
ac
where B is the base and BH
is termed the
conjugate acid
of the base.
The equilibrium constant for this reaction is written as
