Acids and Bases: Ionization Constants

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Introduction

Molecular Structure

Ionization Constants

Salts

Buffers

Lewis Theory

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 (Ka) 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)