Table of Common Ka Values for Weak Acids

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Ka is the equilibrium constant for the dissociation reaction of a weak acid. A weak acid is one which only partially dissociates in water or an aqueous solution. The value of Ka is used to calculate pH of weak acids. The pKa value is used to choose a buffer when needed. Choosing an acid or base where pKa is close to the pH needed gives the best results.

Relating pH, Ka, and pKa

pH, Ka, and pKa are all related to each other. For an acid HA:

Ka = [H+][A-] / [HA]
pKa = - log Ka
pH = - log([H+])

At the halfway point on an equivalence curve, pH=pKa

Ka of Weak Acids

Name Formula Ka pKa
acetic HC2H3O2 1.8 x 10-5 4.7
ascorbic (I) H2C6H6O6 7.9 x 10-5 4.1
ascorbic (II) HC6H6O6- 1.6 x 10-12 11.8
benzoic HC7H5O2 6.4 x 10-5 4.2
boric (I) H3BO3 5.4 x 10-10 9.3
boric (II) H2BO3- 1.8 x 10-13 12.7
boric (III) HBO32- 1.6 x 10-14 13.8
carbonic (I) H2CO3 4.5 x 10-7 6.3
carbonic (II) HCO3- 4.7 x 10-11 10.3
citric (I) H3C6H5O7 3.2 x 10-7 6.5
citric (II) H2C6H5O7- 1.7 x 105 4.8
citric (III) HC6H5O72- 4.1 x 10-7 6.4
formic HCHO2 1.8 x 10-4 3.7
hydrazidic HN3 1.9 x 10-5 4.7
hydrocyanic HCN 6.2 x 10-10 9.2
hydrofluoric HF 6.3 x 10-4 3.2
hydrogen peroxide H2O2 2.4 x 10-12 11.6
hydrogen sulfate ion HSO4- 1.2 x 10-2 1.9
hypochlorous HOCl 3.5 x 10-8 7.5
lactic HC3H5O3 8.3 x 10-4 3.1
nitrous HNO2 4.0 x 10-4 3.4
oxalic (I) H2C2O4 5.8 x 10-2 1.2
oxalic (II) HC2O4- 6.5 x 10-5 4.2
phenol HOC6H5 1.6 x 10-10 9.8
propanic HC3H5O2 1.3 x 10-5 4.9
sulfurous (I) H2SO3 1.4 x 10-2 1.85
sulfurous (II) HSO3- 6.3 x 10-8 7.2
uric HC5H3N4O3 1.3 x 10-4 3.9