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112 Special Topic I
Copyright © 2017 Pearson Education, Inc.
Strong Acids
A strong acid dissociates completely in solution. Strong acids have
p
K
a
values
6
1.
Because a strong acid dissociates completely, the concentration of hydrogen ions is the same as the concentration
of the acid: a 1.0 M HCl solution contains 1.0 M
3
H
+
4
;
a 1.5 M HCl solution contains 1.5 M
3
H
+
4
.
Therefore, to determine the pH of a strong acid, the
3
H
+
4
value does not have to be calculated; it is the same as the
molarity of the strong acid.
Solution
H
pH
1.0 M HCl
1.0 10 M HCl
M
6.4
2
4
[ ]
.
+
M
0
.
.
−
−
−
×
×
×
1 0
1 0 10
2 0
10
2
M HCl
6.4 10 M 3.2
4
×
−
Strong Bases
Strong bases are compounds such as NaOH or KOH that dissociate completely in water.
Because they dissociate completely, the concentration of hydroxide ion is the same as the concentation of the
strong base.
pOH describes the basicity of a solution. The smaller the pOH, the more basic the solution; just like the smaller the
pH, the more acidic the solution.
pOH
=
-
log
3
HO
-
4
3
HO
-
4
and
3
H
+
4
are related by the ionization constant for water
1
K
w
2
.
K
w
14
[H ] [HO ] 10
pH pOH 14
=
=
+
=
+
−
−
Solution
[HO ]
pOH
pH
1.0 M NaOH
1.0 M
0
14.0 0 14.0
1.0 10 M NaOH
4
−
− =
−
×
1.0 10 M 4.0
14.0 4.0 10.0
7.8 10 M NaOH 7.8 10 M 1.1
14.0 1.
4
2
2
×
− =
×
−
−
−
−
×
1 12.9
=
Weak Acids
A weak acid does not dissociate completely in solution. Therefore,
3
H
+
4
must be calculated to determine the pH.