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Homework Help: (Chemistry) Ka of unknown acid by titrations.

  1. Nov 11, 2013 #1
    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?
    Last edited: Nov 11, 2013
  2. jcsd
  3. Nov 11, 2013 #2


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