202 Chapter 7 Acids and Bases
Table 7.2
Values of Ka for Some Common Monoprotic Acids ◀
Name
Value of Ka
HSO42
HClO2
HC2H2ClO2
HF
HNO2
HC2H3O2
[Al(H2O)6]31
HOCl
HCN
NH41
HOC6H5
Hydrogen sulfate ion
Chlorous acid
Monochloracetic acid
Hydrofluoric acid
Nitrous acid
Acetic acid
Hydrated aluminum(III) ion
Hypochlorous acid
Hydrocyanic acid
Ammonium ion
Phenol
1.2 3 1022
1.2 3 1022
1.35 3 1023
7.2 3 1024
4.0 3 1024
1.8 3 1025
1.4 3 1025
3.5 3 1028
6.2 3 10210
5.6 3 10210
1.6 3 10210
Increasing acid strength
Formula
888888888888888888888n
Appendix Table A5.1 contains Ka values.
INTERACTIVE Example 7.1
Relative Base Strength
Using Table 7.2, arrange the following species according to their strength as
bases: H2O, F2, Cl2, NO22, and CN2.
Solution Remember that water is a stronger base than the conjugate base of a
strong acid but a weaker base than the conjugate base of a weak acid. This rule
leads to the following order:
Cl2 , H2O , conjugate bases of weak acids
Weakest bases 888888n Strongest bases
We can order the remaining conjugate bases by recognizing that the strength
of an acid is inversely related to the strength of its conjugate base. From Table
7.2 we have
Ka for HF . Ka for HNO2 . Ka for HCN
Thus the base strengths increase as follows:
F2 , NO22 , CN2
The combined order of increasing base strength is
Cl2 , H2O , F2 , NO22 , CN2
Water as an Acid and a Base
A substance is said to be amphoteric if it can behave either as an acid or as a
base. Water is the most common amphoteric substance. We see this behavior in
the autoionization of water, which involves the transfer of a proton from one
water molecule to another to produce a hydroxide ion and a hydronium ion:
S
S
S
H
G
O
O
+
D
D G
H
H H
S
O
?%/
H H H
+
OOH
+ SO
Q
−
In this reaction one water molecule acts as an acid by furnishing a proton, and
the other acts as a base by accepting the proton. This reaction also can be
represented as follows:
+
+
+
–
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