pH = -log10[H+]
Measure or determine the hydrogen ion concentration, [H+], in moles per liter
Take the base-10 logarithm of [H+]
Multiply the result by -1
If given pOH, use pH = 14 – pOH
If given hydroxide ion concentration, [OH-], use pOH = -log10[OH-], then pH = 14 – pOH
For strong acids, assume complete dissociation to find [H+]
For strong bases, find [OH-] from dissociation, then convert to pH
For weak acids or bases, use the appropriate equilibrium expression to find [H+] or [OH-] first
For pure water at 25°C, pH = 7
For acidic solutions, pH < 7
For basic solutions, pH > 7