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 + H 2O ⇆ A - + H 3O +

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:

K a = [A -][H 3O +] / [HA][H 2O]

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 +
K a = [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)
K a = [H +][B -] / [HB]

For the dissociation of ethanoic acid:

CH 3COOH (aq) + H 2O (l) = CH 3COO - (aq) + H 3O + (aq)
K a = [CH 3COO - (aq)][H 3O + (aq)] / [CH 3COOH (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:

CH 3CH 2CO 2H + H 2 ⇆ H 3O + + CH 3CH 2CO 2 -

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:

  CH 3CH 2CO 2H H 3O + CH 3CH 2CO 2 -
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 = [H 3O +

Now use the pH formula:

pH = -log[H 3O +]
-pH = log[H 3O +] = 4.88
[H 3O + = 10 -4.88 = 1.32 x 10 -5

Plug in this value for x to solve for Ka:

K a = [H 3O +][CH 3CH 2CO 2 -] / [CH 3CH 2CO 2H]
K a = x 2 / (0.2 - x)
K a = (1.32 x 10 -5) 2 / (0.2 - 1.32 x 10 -5)
K a = 8.69 x 10 -10
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Helmenstine, Anne Marie, Ph.D. "Acid Dissociation Constant Definition: Ka." ThoughtCo, Aug. 26, 2020, thoughtco.com/acid-dissociation-constant-definition-ka-606347. Helmenstine, Anne Marie, Ph.D. (2020, August 26). Acid Dissociation Constant Definition: Ka. Retrieved from https://www.thoughtco.com/acid-dissociation-constant-definition-ka-606347 Helmenstine, Anne Marie, Ph.D. "Acid Dissociation Constant Definition: Ka." ThoughtCo. https://www.thoughtco.com/acid-dissociation-constant-definition-ka-606347 (accessed July 26, 2021).