Science, Tech, Math › Science Table of Common Ka Values for Weak Acids Share Flipboard Email Print Thomas Barwick/Getty Images Science Chemistry Periodic Table Basics Chemical Laws Molecules Projects & Experiments Scientific Method Biochemistry Physical Chemistry Medical Chemistry Chemistry In Everyday Life Famous Chemists Activities for Kids Abbreviations & Acronyms Biology Physics Geology Astronomy Weather & Climate by 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. Updated September 25, 2019 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 KapH = - 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 Continue Reading Understand the Relationship Between pH and pKa Chemistry Basics: What Is pKa? Definitions of pH, pKa, Ka, pKb, and Kb What the Acid Dissociation Constant Is and How to Calculate It Practice Calculating the pH of a Weak Acid With This Sample Problem What Is Titration? See Titration Curves What Is a Weak Acid? Common Strong and Weak Acids and Ways to Tell Them Apart Know What a Strong Acid Is in Chemistry Practice Calculating the pH of a Strong Acid with This Sample Problem How to Make a Phosphate Buffer Solution Key Acids and Bases Definitions Why Hydrofluoric Acid Is a Weak Acid Learn The Chemistry Behind Salt Formation What Is the Henderson Hasselbalch Equation? What Do You Need to Know to Calculate pH?