5 Steps to a 5 AP Chemistry

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 15.1KaValues for Selected Weak Acids

NAME (FORMULA) LEWIS STRUCTURE Ka

Iodic acid (HIO 3 ) 1.6 × 10 −^1

Chlorous acid (HClO 3 ) 1.12 × 10 −^2

Nitrous acid (HNO 2 ) 7.1 × 10 −^4

Hydrofluoric acid (HF) 6.8 × 10 −^4

Benzoic acid (C 6 H 5 COOH) 6.3 × 10 −^5

Acetic acid (CH 3 COOH) 1.8 × 10 −^5

Propanoic acid (CH 3 CH 2 COOH) 1.3 × 10 −^5

Hypochlorous acid (HClO) 2.9 × 10 −^8

Hypobromous acid (HBrO) 2.3 × 10 −^9

Phenol (C 6 H 5 OH) 1.0 × 10 −^10

Hypoiodous acid (HIO) 2.3 × 10 −^11

If the initial molarity and Kaof the weak acid are known, the [H+] (or [A−]) can be

calculated easily. And if the initial molarity and [H+] are known, Kacan be calculated.

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

Ka

232

+

HC H O aq) H aq C H O aq) 232

0.300

=×

+

−

−

−

18 10.^5

(() (

xxx x

K

x

a

+

232

232

HCHO

HC H O

)(

==×

=

[][ ] −

[]

.

(

18 10^5

xx

x

x

)

[H+ M

0 300

18 10

23 10

5

3

.

.

].

−

=×

==×

−

−

H O I

O H

H O Br

H O Cl

H C

H

H

C

H

H

C O H

O

H C

H

H

C O H

O

C O H

O

H F

H O N O

H O Cl O

H O I O

O

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