pH can be calculated using the mathematical equation when the concentrations of hydrogen ions are know:
pH = -log[H+]
Example: In a solution of 0.5M HCl pH = -log[0.5] = 0.301 ∴ HCl is acidic
Ionic product of water:
Kw = [H3O+][OH–] = 1 × 10-14
Using the ionic product of water, we can calculate the concentrations of hydronium ions or concentration of hydroxide ions when one is know.
Example: Calculate [H3O+] when [OH–] = 1 × 10-8, Kw = [H3O+][OH–] 1 × 10-14 = [H3O+][1 × 10-8] [H3O+] = 1 × 10-6
Taking log on the ionic product of water, the following equation is obtained:
pH + pOH = 14 (where pOH = -log[OH–])
Example: Calculate the pOH when pH = 4, pH + pOH = 14 4 + pOH = 14 pOH = 10