Chemistry Reference
In-Depth Information
Taking the square root of both sides of the equation yields
10
3
x
1.68
Because
x
represents both the hydronium ion concentration and the acetate ion
concentration, both of these are equal to Finally,
is about 1% of 0.160 M, so the approximation is acceptable.
10
3
M.
10
3
M
1.68
1.68
Practice Problem 19.7
Calculate the hydroxide ion concentration that
results when 0.160 mol of ammonia is dissolved in enough water to make 1.00 L
of solution.
Snapshot Review
❒
The ionization constant expression has no term for the concentration
of water because it is used for dilute aqueous solutions only, in which
the concentration of water is constant.
ChemSkill Builder
18.1, 18.5
A. Calculate the concentration of ammonium ion in 0.0165 M aqueous
ammonia.
10
5
K
b
1.77
19.3
Autoionization of Water
We learned in Section 19.1 that water can act as either an acid or a base. In
fact, water can act as both an acid and a base
in the same reaction:
H
2
O(
/
) E H
3
O
(aq)
OH
(aq)
H
2
O(
/
)
This reaction, which is an example of
autoionization,
proceeds to the right to
only a very tiny extent because water is such a weak acid and base. To write
an equilibrium constant expression for this reaction, we again consider the con-
centration of water itself to be a constant, which we incorporate into the value
of
K
. We get another type of constant, called the
water ionization constant
and
denoted
K
w
:
[H
3
O
][OH
]
K
w
Note that this equilibrium constant expression has no denominator and also that
the corresponding chemical equation is written with water as the reactant (on
the left). The value of for dilute aqueous solutions at is
Because the product is a constant, the concentration of hydro-
nium ion in any dilute aqueous solution can be calculated from the concentra-
tion of hydroxide ion, and vice versa.
[
H
3
O
][
OH
]
K
w
10
14
.
K
w
25°C
1.00
1.00 10
14
[H
3
O
][OH
]
EXAMPLE 19.8
Calculate the hydronium ion concentration of a solution in which the hydrox-
ide ion concentration is 6.15
10
2
M.
