5 Steps to a 5 AP Chemistry 2019

Equilibrium ❮ 219

is formed, an HA must have dissociated. However, many times if Ka is small, you can
approximate the equilibrium concentration of the weak acid by its initial concentration,
[HA] = Minitial.

Table 15.1 Ka Values for Selected Weak Acids

NAME (FORMULA) LEWIS STRUCTURE Ka

Iodic acid (HIO 3 )

H O I O

O^ 1.6 × 10 -^1

Chlorous acid (HClO 2 ) H O Cl O 1.12 × 10 -^2

Nitrous acid (HNO 2 ) H O N O 7.1 × 10 -^4

Hydrofluoric acid (HF) H F 6.8 × 10 -^4

Benzoic acid (C 6 H 5 COOH)
C O H

O

6.3 × 10 -^5

Acetic acid (CH 3 COOH)

H C

H

H

C O H

O

1.8 × 10 -^5

Propanoic acid (CH 3 CH 2 COOH)

H C

H

H

C

H

H

C O H

O

1.3 × 10 -^5

Hypochlorous acid (HClO) H O Cl 2.9 × 10 -^8

Hypobromous acid (HBrO) H O Br 2.3 × 10 -^9

Phenol (C 6 H 5 OH) O H 1.0 × 10 -^10

Hypoiodous acid (HIO) H O I 2.3 × 10 -^11

If the initial molarity and Ka of the weak acid are known, the [H+] (or [A-]) can be
calculated easily. And if the initial molarity and [H+] are known, Ka can be calculated.

For example, calculate the [H+] of a 0.300 M acetic acid solution.



=×

+

−

==×

=

−

=×

==×

−

+−

+

−

−

+−

K

xx x

K

xx

x

x

1.8 10

HC HO(aq) H(aq)CHO(aq)

0.300

[H ][CHO]

[HCHO]

1.8 10

()()

0.300

1.8 10

[H ] 2.3 10

a

5

232232

a

232

232

5

5

(^3) M