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.
Henderson-Hasselbalch pH formula for a buffer solution is:
pH = pKa + log([base] / [acid])
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?