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Homework Help: Identity of a Weak Acid

  1. Mar 22, 2015 #1
    1. The problem statement, all variables and given/known data

    What is the likely identity of a weak acid if a 0.50 mol/L solution of the acid has a pH of 3.18?

    2. Relevant equations

    HA + H2O <=> H3O + A

    [tex] K_{a} = \frac{[P]}{[R]} [/tex]

    [tex] [H3O] = 10^{-pH} [/tex]

    3. The attempt at a solution

    I set up an ICE table with the concentration given, then solved for x using the inverse log formula for [H3O], finally I did the following Ka expression:

    [tex] K_{a} = \frac{(6.6 \times 10^{-4})^{2}}{0.5-6.6 \times 10^{-4}} = 8.75 \times 10^{-7} [/tex]

    The closets acid to that appears to be carbonic acid (ka = 4.4x10^-7). Any suggestions?
  2. jcsd
  3. Mar 23, 2015 #2


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    Science Advisor
    Homework Helper
    Gold Member

    You've doubly ionized a diprotic weak acid at a pH of 3.2? Try it again.
  4. Mar 23, 2015 #3


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    Staff: Mentor

    I am not sure what you mean, Ka is calculated assuming the acid behaves as a monoprotic. That's not a bad assumption for a carbonic acid (no idea if the OP was aware of the problem, but that's another story).

    Sadly, pKa of 6.05 doesn't fit any acid commonly used in introductory chemistry courses. Of those used carbonic acid looks best (but still is way off with the pKa1=6.37).
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