Calculate the acid dissociation constant from concentration and measured pH.
For a weak monoprotic acid the dissociated proton and conjugate base concentrations are equal, so K_a reduces to [H⁺]² divided by the undissociated remainder. Percent dissociation shows how far the approximation of small dissociation holds. pK_a is the single most useful number for an acid because it equals the pH at which the acid is exactly half dissociated.
Ka
K_a = [H⁺]² ÷ (C − [H⁺]) for a monoprotic weak acid
K_a = [H⁺]² ÷ (C − [H⁺]) for a monoprotic weak acid For a weak monoprotic acid the dissociated proton and conjugate base concentrations are equal, so K_a reduces to [H⁺]² divided by the undissociated remainder. Percent dissociation shows how far the approximation of small dissociation holds.
pK_a is the single most useful number for an acid because it equals the pH at which the acid is exactly half dissociated.
This calculator takes 2 inputs: Initial acid concentration, Measured pH. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.