1. The problem statement, all variables and given/known data Let's say that i have a unknown weak acid. I prepare 10mL of the weak acid diluted in 25mL of water. I titrate the solution with NaOH. 7.37mL of NaOH was needed for a full titration. Therefore, half-titration of this acid was 3.69mL. I measure the pH of the solution with a pH-meter and the pH was 4.7. I want to find the Ka of the unknown acid. 2. Relevant equations Henderson-Hasselbalch pH formula for a buffer solution is: pH = pKa + log([base] / [acid]) 3. The attempt at a solution Since the weak acid is monoprotic, the half titration of the solution gives an equal concentration of both acid and conjugate base. pH = pKa + log(1) = pKa Does it mean that Ka is the concentration of [H+]? Ka = 10^-pH = 10^-4.7 = 2.0 e-5? Edit: Is it possible to calculate in a different way using the given data above? Ka = [H+][A-]/[HA] I can calculate the concentration of [H+] from the pH but i don't know the initial concentration of [HA]. And what about the concentration of [A-] ion?