Fraction of Association of an Acid?

Calculate the fraction of association of a 0.30 F benzoic acid solution.

So, I know that you basically just need to find what [A-] is and divide that by .30 (which I found to be .0144). However, technically that is the fraction of dissociation. Can you find the fraction of association for an acid? I think I'm really having difficulty understanding the difference between fraction of association vs. fraction of dissociation. Can anybody help?

1 Answer

  • Ka of benzoic acid = 6.3 × 10-5

    Calculate [H+] using Ka equation:

    Ka = [H+]² / [acid]

    6.3*10^-5 = [H+]² / 0.30

    [H+]² = (6.3*10^-5)*0.3

    [H+]² = 1.89*10^-5

    [H+] = 4.35*10^-3

    % dissociation = [H+] / [acid] *100

    Because the dissociation is small we can call [acid] = 0.30M

    %dissociation = (4.35*10^-3) / 0.3 *100

    % dissociation = 1.45%

    Therefore % association = 100 - 1.45 = 98.55%

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