Weak Acids and Bases, Ka for weak acids 

Calculating the pH of strong acids is straightforward - the acid dissociates completely, and the H+ ion concentration is equal to the acid concentration. Calculating the pH of weak acids is not as straightforward. Weak acids only dissociate partially and the H+ ion concentration is much smaller than the acid concentration. Here, [HA] is the concentration of the undissociated acid and the ‘products’ are the ions formed from dissociation:

To calculate the H+ ion concentration of a weak acid, we use the acid dissociation constant (Ka) and the equation:

Two assumptions are made when using this equation:

1. The concentration of hydrogen ions is equal to the concentration of the anions

2. The concentration of undissociated acid, HA, at equilibrium is unchanged

Assumption 2 implies:

Using the concentration values of acids is a little unwieldy – it is sometimes better to use pKa. pKa is another measure of the strength of an acid - the lower the value, the stronger the acid and the greater its ability to donate its protons. The relationship between pKa and Ka is:

Examples:

Write an expression for the acid dissociation constant Ka for ethanoic acid.

For this dissociation, we have:

The expression is, therefore:

We are given the pH so can find the hydrogen ion concentration:

Therefore:

Using the acid dissociation constant:

rearranging gives: