Equilibria of Weak Acids, Ka

What is a Weak Acid?

A weak acid is any acid that reacts with water (donates H+ ions) to a very small extent, usually less than 5 - 10%.  An aqueous solution of a weak acid in a state of equilibrium would consist mainly of the unionized form of the acid, and only a small amount of hydronium ions and of the anion (conjugate base) of the weak acid.  The equation representing the ionization of any weak acid, HA, and the equilibrium expression, Ka, are shown below.

Equation and Equilibrium Expression for HA

Note: This expression, Ka, is based on the general form for Kc. The designation Ka is used to indicate that it is the equilibrium constant for the reaction of an acid with water.


Calculating Ka

To calculate the ionization constant, Ka, you need to know:

To find Ka substitute the values for the equilibrium concentrations into the equilibrium expression and solve for Ka.

Example:  The pH of a 0.1000 M solution of acetylsalicylic acid (aspirin-"HAsp") was found to be 2.24.  Determine the value of Ka, the ionization constant for acetylsalicylic acid, Ka.  The formula for acetylsalicylic acid is CH3CO2C6H4COOH, but we use "HAsp" as an abbreviation.

Calculation of K<sub>a</sub >


Calculating the Equilibrium Concentrations in an Aqueous Solution of a Weak Acid

To calculate the equilibrium concentrations you need to know:

To determine the equilibrium concentrations: Example:  Calculate the concentrations for each species present in a 0.1000 M aqueous solution of nitrous acid (Ka = 6.0 x 10-4).