Acids and Bases: Ionization Constants
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Introduction
Molecular Structure
Ionization Constants
Salts
Buffers
Lewis Theory

Step 2: Determine how the initial concentrations will change when ionization occurs.

During ionization, some of the lactic acid will ionize, forming some lactate ion and hydronium ion. At this point, the amount of ionization is not known so the variable x can be used to represent the extent of ionization.

If the concentration of lactic acid is decreased by x, what will be the change in concentration of lactate ion?

+ 1/2 x + x + 2x

The stoichiometry of the chemical equation shows when one mole of lactic acid ionizes, one mole of lactate ion is formed.

Good! This information can be added to the table:

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












Step 3: Determine what is present in the solution when equilibrium is reached.

While it may seem no information is known about the solution after ionization, a pH of 2.08 was measured. Recall that the pH is a measure of the H+ concentration.

What is the H+ concentration in this solution at equilibrium?

.0083 .32 2.08

pH = - log [ H+ ]

This can be written as:

[ H+ ] = 10-pH

With this information, the value of the variable x can be determined:

Initial + Change = Equilibrium
1 x 10-7 M +   (+ x)  =   0.0082999 M 
x = 0.0083 M

You can see that when significant figures are taken into account, the the H+ from the autoionization of water (1 x 10-7 M) is not significant.

With the value of x now known, the table can be completed.

Initial 
 		 0.50 M 
 		 0 
 		 1 x 10-7 M
(from water)
Change - 0.0083 M + 0.0083 M + 0.0083 M
Equilibrium 0.49 M 0.0083 M 0.0083 M

With the table completed, enough information is known to calculate Ka for lactic acid.