Table of Common Ka Values for Weak Acids

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Todd Helmenstine
Todd Helmenstine
Todd Helmenstine is a science writer and illustrator who has taught physics and math at the college level. He holds bachelor's degrees in both physics and mathematics.
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Updated on February 03, 2020

Ka is the equilibrium constant for the dissociation reaction of a weak acid. A weak acid is one that only partially dissociates in water or an aqueous solution. The value of Ka is used to calculate the 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

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
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Your Citation
Helmenstine, Todd. "Table of Common Ka Values for Weak Acids." ThoughtCo, Apr. 5, 2023, thoughtco.com/acids-and-bases-weak-acid-ka-values-603973. Helmenstine, Todd. (2023, April 5). Table of Common Ka Values for Weak Acids. Retrieved from https://www.thoughtco.com/acids-and-bases-weak-acid-ka-values-603973 Helmenstine, Todd. "Table of Common Ka Values for Weak Acids." ThoughtCo. https://www.thoughtco.com/acids-and-bases-weak-acid-ka-values-603973 (accessed June 5, 2023).
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