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 pH Calculations of Weak Acids

Basic Information

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