# How do you calculate the pH of a weak acid?

##### 2 Answers

A handy expression to use is

#### Explanation:

This is a quick and useful method if you are given the

I'll derive it using the usual ICE method:

If

We assume that

Taking -ve logs of both sides

Here's an example:

If you can remember it then fine. If not you need to use the ICE method described in the answer by @Dr Hayek

#### Explanation:

By measuring the concentration of

Consider the dissociation of the following weak acid HA with initial concentration of

The expression of

solve for

that can be rounded to

Here is a video which summarizes this technique which requires use of the quadratic formula. The values are different than Dr. Hayek's example, but the technique is the same.

Video from: Noel Pauller