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 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 K a pK a
acetic HC 2H 3O 2 1.8 x 10 -5 4.7
ascorbic (I) H 2C 6H 6O 6 7.9 x 10 -5 4.1
ascorbic (II) HC 6H 6O 6 - 1.6 x 10 -12 11.8
benzoic HC 7H 5O 2 6.4 x 10 -5 4.2
boric (I) H 3BO 3 5.4 x 10 -10 9.3
boric (II) H 2BO 3 - 1.8 x 10 -13 12.7
boric (III) HBO 3 2- 1.6 x 10 -14 13.8
carbonic (I) H 2CO 3 4.5 x 10 -7 6.3
carbonic (II) HCO 3 - 4.7 x 10 -11 10.3
citric (I) H 3C 6H 5O 7 3.2 x 10 -7 6.5
citric (II) H 2C 6H 5O 7 - 1.7 x 10 5 4.8
citric (III) HC 6H 5O 7 2- 4.1 x 10 -7 6.4
formic HCHO 2 1.8 x 10 -4 3.7
hydrazidic HN 3 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 H 2O 2 2.4 x 10 -12 11.6
hydrogen sulfate ion HSO 4 - 1.2 x 10 -2 1.9
hypochlorous HOCl 3.5 x 10 -8 7.5
lactic HC 3H 5O 3 8.3 x 10 -4 3.1
nitrous HNO 2 4.0 x 10 -4 3.4
oxalic (I) H 2C 2O 4 5.8 x 10 -2 1.2
oxalic (II) HC 2O 4 - 6.5 x 10 -5 4.2
phenol HOC 6H 5 1.6 x 10 -10 9.8
propanic HC 3H 5O 2 1.3 x 10 -5 4.9
sulfurous (I) H 2SO 3 1.4 x 10 -2 1.85
sulfurous (II) HSO 3 - 6.3 x 10 -8 7.2
uric HC 5H 3N 4O 3 1.3 x 10 -4 3.9
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Your Citation
Helmenstine, Todd. "Table of Common Ka Values for Weak Acids." ThoughtCo, Aug. 27, 2020, thoughtco.com/acids-and-bases-weak-acid-ka-values-603973. Helmenstine, Todd. (2020, August 27). 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 April 23, 2021).

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