Calculate the hydrogen-ion concentration with the formula [H+] = 1/(10^pH), where [H+] is the concentration of hydrogen ions. This formula is derived from the definition of pH: pH = -log[H+]. For example, if the pH of a solution of benzoic acid is 2.51, [H+] = 1/(10^2.51) = 3.09 x 10^-3 moles/liter.
Look up the acid's dissociation constant (Ka) on a table of weak acids (see Resource). From the table, the dissociation constant for benzoic acid is Ka = 6.46 x 10^-5.
Calculate the initial concentration of the acid. By definition, the dissociation constant is Ka = [H+][A-]/[HA], where [HA] is the initial concentration, and [A-] is the concentration of the acid's anions, which are negatively charged ions. At equilibrium, [HA] will decrease by an amount equal to [H+], and [H+] is also equal to [A-]. Therefore, you can rewrite the expression as Ka = [H+]²/([HA] - [H+]). Solve for [HA] to derive the formula [HA] = [H+]²/Ka + [H+]. For example: [HA] = (3.09 x 10^-3)²/(6.46 x 10^-5) + (3.09 x 10^-3) = 0.151 moles/liter.
Find the percentage of ionization with the formula I = 100*[H+]/[HA]. For example, I = 100*(3.09 x 10^-3)/0.151 = 2.05 percent.