Acids and Bases: Ionization Constants

Table of Contents

Molecular Structure

Ionization Constants

How are ionization constants determined?

One method of determining the ionization constant for an acid or base is to measure the pH of a solution of known concentration. Working through the following example will show you how this is done.

A 0.50 M lactic acid (HC₃H₅O₃) solution was made by dissolving 45 grams lactic acid in water and diluting to one liter. The pH of this solution is found to be 2.08. What is the acid ionization constant (K_a) for lactic acid?

The equilibrium constant for the following reaction is to be determined:

Step 1: Determine what is present in the solution initially (before any ionization occurs).

Will HC₃H₅O₃ be present initially?		Yes	No
Will C₃H₅O₃^- be present initially?		Yes	No
Will H⁺ be present initially?		Yes	No

Before ionization occurs, HC₃H₅O₃ will be present.

C₃H₅O₃ will not be present until some ionization has occurred.

While no H⁺ will be present from the ionization of HC₃H₅O₃, this is an aqueous solution so there will be an H⁺ concentration of 1 x 10^-7 due to the autoionization of water.

It is helpful to put this information into an ICE (Initial, Change, Equilibrium) table:


Initial	0.50 M		0		1 x 10^-7 M (from water)

Good!

Good!

It is helpful to put this information into a table:


Initial	0.50 M		0		1 x 10^-7 M (from water)