1) Weak acids are less than 100% ionized in solution. They have a small Ka value. 2) Acetic acid (formula = HC2H3O2) is the most common weak acid example used by instructors. 3) Another way to write acetic acid's formula is CH3COOH. 4) A common abbreviation for acetic acid is HAc, where Ac¯ refers to the acetate polyatomic ion.
Ka of Weak Acids
Name
Formula
Ka
acetic
HC2H3O2
1.8 x 10-5
ascorbic (I)
H2C6H6O6
7.9 x 10-5
ascorbic (II)
HC6H6O6-
1.6 x 10-12
benzoic
HC7H5O2
6.4 x 10-5
boric (I)
H3BO3
5.4 x 10-10
boric (II)
H2BO3-
1.8 x 10-13
boric (III)
HBO32-
1.6 x 10-14
carbonic (I)
H2CO3
4.5 x 10-7
carbonic (II)
HCO3-
4.7 x 10-11
citric (I)
H3C6H5O7
3.2 x 10-7
citric (II)
H2C6H5O7-
1.7 x 105
citric (III)
HC6H5O72-
4.1 x 10-7
formic
HCHO2
1.8 x 10-4
hydrazidic
HN3
1.9 x 10-5
hydrocyanic
HCN
6.2 x 10-10
hydrofluoric
HF
6.3 x 10-4
hydrogen peroxide
H2O2
2.4 x 10-12
hydrogen sulfate ion
HSO4-
1.2 x 10-2
hypochlorous
HOCl
3.5 x 10-8
lactic
HC3H5O3
8.3 x 10-4
nitrous
HNO2
4.0 x 10-4
oxalic (I)
H2C2O4
5.8 x 10-2
oxalic (II)
HC2O4-
6.5 x 10-5
phenol
HOC6H5
1.6 x 10-10
propanic
HC3H5O2
1.3 x 10-5
sulfurous (I)
H2SO3
1.4 x 10-2
sulfurous (II)
HSO3-
6.3 x 10-8
uric
HC5H3N4O3
1.3 x 10-4
An other weak acid example
Vinegar is a dilute water solution of acetic acid with small amounts of other components. Calculate the pH of bottled vinegar that is 0.667 M HC2H3O2, assuming that none of the other components affect the acidity of the solution.
HC2H3O2(aq) H+(aq) + C2H3O2-(aq)
We get the value for the acid dissociation constant for this reaction from the table above.
**x in the denominator, is considered too small compared to 0.667, so it is ignored.
x2 = 1.2 x 10-5 x = 3.5 x 10-3
[H+] = 3.5 x 10-3 M H+ pH = -log(3.5 x 10-3) = 2.46
H+(aq) + C2H3O2-(aq)
