7.2.12 Buffer Solutions - Calculations

Expression for Acidic Buffer

Consider an acidic buffer made using a weak acid and its conjugate base , at equilibrium:

The acid dissociation constant () for this equilibrium is given by:

Rearranging the equation to make subject:

Taking logs on both sides, we get the Henderson-Hasselbalch Equation:

Critical: and are NOT the same

• Recall that when finding the pH of a weak acid, we assumed that .
• However, this assumption is incorrect for a buffer solution.
• Since the concentration of comes from the salt (e.g. Sodium Ethanoate) rather than the acid, therefore the concentration of is much higher than .