Identify the concentration of hydronium ions in an acid solution. For example, assume it is given that a particular acid has a hydronium ion concentration equal to 1.2 x 10^-4 M. The unit, M, refers to molarity which is identified as the moles per liter of a molecule in solution. A large molarity indicates a greater concentration of molecules in solution.
Calculate the pH of the acid by taking the negative logarithm of the hydronium ion concentration. This is done by plugging the expression into a scientific calculator. Thus, the pH of an acid with a hydronium ion concentration of 1.2 x 10^-4 M would be 3.9.
pH = -log[H3O+]
pH = -log[1.2 x 10^-4]
pH = 3.9
Realize that as the hydronium ion concentration increases, the pH will subsequently decrease thereby indicating a stronger acid. The strength of an acid is directly related to how well it dissociates in solution.