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(Chemistry) Ka of unknown acid by titrations.

  • Thread starter cshum00
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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?
 
Last edited:

Answers and Replies

  • #2
Borek
Mentor
28,441
2,838
Yes.
 

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