IB Chemistry home > Syllabus 2016 > Acids and Bases > Measuring acidity

Syllabus ref: 8.3

The acidity of a solution is dependent on the concentration of hydrogen ions. The pH scale measures this acidity.

Nature of science:

Occam's razor: the pH scale is an attempt to scale the relative acidity over a wide range of H+ concentrations into a very simple number.


Essential idea: The pH scale is an artificial scale used to distinguish between acid, neutral and basic/alkaline solutions.

pH = - log[H+(aq)] and [H+] = 10-pH.

A change of one pH unit represents a 10-fold change in the hydrogen ion concentration [H+].

pH values distinguish between acidic, neutral and alkaline solutions.

The ionic product constant, Kw = [H+][OH-] = 10-14 at 298 K.

Applications and skills

Solving problems involving pH, [H+] and [OH-].

Students should be familiar with the use of a pH meter and universal indicator.

In Chapter 8.3