Weak Acids

It is a bit trickier to calculate the pH of a weak acid. This is because they only partially dissociate. However, this means that these acids have an equilibrium constant (called the acid dissociation constant), which can be used to calculate the hydronium ion concentration:

The process for doing this is:

1. Write the balanced equation for the dissociation with water.
2. Write the acid dissociation expression.
3. Substitute and solve
• [hydronium] = [conjugate base] = x
• [acid] = original concentration stated in the question (only partially dissociated)
4. Calculate pH