Science, Tech, Math › Science Acid Dissociation Constant Definition: Ka What Is an Acid Dissociation Constant, or Ka in Chemistry? Share Flipboard Email Print Maartje van Caspel/Getty Images Science Chemistry Basics Chemical Laws Molecules Periodic Table 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 Anne Marie Helmenstine, Ph.D. Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels. Updated May 25, 2019 The acid dissociation constant is the equilibrium constant of the dissociation reaction of an acid and is denoted by Ka. This equilibrium constant is a quantitative measure of the strength of an acid in a solution. Ka is commonly expressed in units of mol/L. There are tables of acid dissociation constants, for easy reference. For an aqueous solution, the general form of the equilibrium reaction is: HA + H2O ⇆ A- + H3O+ where HA is an acid which dissociates in the conjugate base of the acid A- and a hydrogen ion that combines with water to form the hydronium ion H3O+. When the concentrations of HA, A-, and H3O+ no longer change over time, the reaction is at equilibrium and the dissociation constant may be calculated: Ka = [A-][H3O+] / [HA][H2O] where the square brackets indicate concentration. Unless an acid is extremely concentrated, the equation is simplified by holding the concentration of water as a constant: HA ⇆ A- + H+Ka = [A-][H+]/[HA] The acid dissociation constant is also known as the acidity constant or acid-ionization constant. Relating Ka and pKa A related value is pKa, which is the logarithmic acid dissociation constant: pKa = -log10Ka Using Ka and pKa To Predict Equilibrium and Strength of Acids Ka may be used to measure the position of equilibrium: If Ka is large, the formation of the products of the dissociation is favored.If Ka is small, the undissolved acid is favored. Ka may be used to predict the strength of an acid: If Ka is large (pKa is small) this means the acid is mostly dissociated, so the acid is strong. Acids with a pKa less than around -2 are strong acids.If Ka is small (pKa is large), little dissociation has occurred, so the acid is weak. Acids with a pKa in the range of -2 to 12 in water are weak acids. Ka is a better measure of the strength of an acid than pH because adding water to an acid solution doesn't change its acid equilibrium constant, but does alter the H+ ion concentration and pH. Ka Example The acid dissociation constant, Ka of the acid HB is: HB(aq) ↔ H+(aq) + B-(aq)Ka = [H+][B-] / [HB] For the dissociation of ethanoic acid: CH3COOH(aq) + H2O(l) = CH3COO-(aq) + H3O+(aq)Ka = [CH3COO-(aq)][H3O+(aq)] / [CH3COOH(aq)] Acid Dissociation Constant From pH The acid dissociation constant may be found it the pH is known. For example: Calculate the acid dissociation constant Ka for a 0.2 M aqueous solution of propionic acid (CH3CH2CO2H) that is found to have a pH value of 4.88. To solve the problem, first, write the chemical equation for the reaction. You should be able to recognize propionic acid is a weak acid (because it's not one of the strong acids and it contains hydrogen). It's dissociation in water is: CH3CH2CO2H + H2 ⇆ H3O+ + CH3CH2CO2- Set up a table to keep track of the initial conditions, change in conditions, and equilibrium concentration of the species. This is sometimes called an ICE table: CH3CH2CO2H H3O+ CH3CH2CO2- Initial Concentration 0.2 M 0 M 0 M Change in Concentration -x M +x M +x M Equilibrium Concentration (0.2 - x) M x M x M x = [H3O+ Now use the pH formula: pH = -log[H3O+]-pH = log[H3O+] = 4.88[H3O+ = 10-4.88 = 1.32 x 10-5 Plug in this value for x to solve for Ka: Ka = [H3O+][CH3CH2CO2-] / [CH3CH2CO2H]Ka = x2 / (0.2 - x)Ka = (1.32 x 10-5)2 / (0.2 - 1.32 x 10-5)Ka = 8.69 x 10-10 Continue Reading Practice Calculating the pH of a Weak Acid With This Sample Problem Find the Ka or Equilibrium Constant Values of Weak Acids Chemistry Basics: What Is pKa? Know What a Strong Acid Is in Chemistry Definitions of pH, pKa, Ka, pKb, and Kb Learn The Chemistry Behind Salt Formation What Is pKb in Chemistry? Key Acids and Bases Definitions What Is a Weak Acid? Practice Calculating the pH of a Strong Acid with This Sample Problem Understand the Relationship Between pH and pKa What Is Titration? See Titration Curves What Do You Need to Know to Calculate pH? Common Strong and Weak Acids and Ways to Tell Them Apart What Is a pH Indicator? Chemistry Basics: What Is a Buffer?