Work out acid dissociation constant instantly with clear inputs, formula shown and shareable results.
For a weak monoprotic acid, the pH gives [H+] directly, and that same figure is the concentration of conjugate base formed. Ka is then [H+]^2 divided by the acid remaining, C - [H+]. A 0.1 M acid reading pH 2.87 has [H+] = 1.35e-3 and Ka near 1.8e-5, the value for acetic acid.
Equilibrium expression
Ka = [H+][A-] / [HA] = [H+]^2 / (C - [H+])
From pH
[H+] = 10^-pH
Because every hydrogen ion released came from an acid molecule, so the undissociated acid left is the initial concentration minus that amount.
When ionisation is under about 5%, the approximation Ka = [H+]^2/C is within experimental error. This calculator does the exact form regardless.