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
218 Step 4. Review the Knowledge You Need to Score High