Weak Acid and Ka

• Weak acids only partially dissociate in water.

• Equilibrium is established between undissociated acid molecules and ions.

For a weak acid, HA, dissociating as:

OR

Unlike strong acids, weak acids do not release all their H⁺ ions in solution.

Ka — Acid Dissociation Constant

Ka is the acid dissociation constant, a measure of the extent of dissociation of a weak acid.

The Ka expression is:

Where:

• = concentration of hydrogen ions

• = concentration of conjugate base

• = concentration of undissociated acid

Units of Ka are typically

The more dissociation a weak acid undergoes, the greater the concentrations of and and therefore the greater the value of Ka.

To compare Ka values more easily, we often use pKa:

To convert back:

Lower pKa = stronger acid (more dissociation)

Higher pKa = weaker acid (less dissociation)

Calculating the pH of a Weak Acid

A few assumptions must be made before using the Ka expression to calculate and therefore pH.

1. Dissociation of the weak acid HA is negligible. This means that and the given acid concentration can be used in calculations.

2. Dissociation of water is negligible. This means that all the ions in the solution come from the acid and not from the partial dissociation of water allowing the simplification ,

This lead to the simplified expression:

This is used to calculate Ka or when dealing with weak monoprotic acids.