Acid Dissociation Constant Definition: Ka

What Is an Acid Dissociation Constant, or Ka in Chemistry?

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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