(Chemistry) Ka of unknown acid by titrations.

[cshum00]

Homework Statement

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.

Homework Equations

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?

 

[Borek]

Yes.