Equations

Law of Mass Action:

To calculate the hydrogen (or hydronium) ion concentration of a solution that contains ONLY the free form, HA of a monoprotic acid:

To calculate the hydroxyl ion concentration of a solution that contains ONLY the ionized form, A^- of a monoprotic acid:

To calculate pk_a when k_a is known:

The Henderson-Hasselbach equation:

Valid when ionization is negligible (<5%), used to calculate the pH of a solution if other information is know, the ratio of ionized to unionized species in solution, or pK_a.

To calculate pH or pOH when either is known:

To calculate pH from the concentration when the concentration of the hydronium ion is known:

To calculate pOH from the concentration when the concentration of the hydroxyl ion is known: