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Stupidity With Acid/Base Work

  1. Oct 27, 2009 #1
    I Need Help With My Acid/Base Work

    Apparently I've caught the stupid virus and absolutely fail at chemistry.

    Todays prelab: 0.010L of .1000M HA with a pKa of 7.00 is titrated with 0.1000M NaOH over a series of volumes:

    L NaOH

    Step 1: Get the pH at zero. Hooray for ICE tables, I got the correct pH of 4.

    Then I set sail for fail with a method my teacher showed us in class.

    So we can see clearly that the equivalence point is at 10ml or 0.010. Oh my! It looks like we can cut everything up into nice neat percentages!

    L NaOH
    .0000 0%
    .0010 10%
    .0020 20%
    .0050 50%
    .0090 90%
    .0098 98%
    .0100 100%

    For the rest of them Kb comes into the mix. Then I did this:

    Ka = [H3O][A]/[HA]

    Well it looks like I can just replace the [A]/[HA] with the percentage as a decimal!

    Ka = [H3O](concentration coefficient)

    Then after some more fun:

    pH = -log(Ka/(concentration coefficient))

    Which is exactly what my teacher did and class for a different setup, and wound up with me getting .25/2 on the pre-lab.

    What went wrong here? Is this even right?
    Last edited: Oct 27, 2009
  2. jcsd
  3. Oct 28, 2009 #2


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    Staff: Mentor

    Re: I Need Help With My Acid/Base Work

    No, percentage is not [A-]/[HA] but [A-]/[analytical concentration of the acid].

  4. Oct 28, 2009 #3
    The way she used it seems to make percentage the percent until completion...

    Here is the slide with the work on it...attachment pending.

    Attached Files:

  5. Oct 28, 2009 #4


    User Avatar

    Staff: Mentor

    Right down: 25/75 is OK, but it is not 25%.

  6. Oct 28, 2009 #5
    ...Ugh. That's a massive fail. Alright, something to remember for next time. So for my situation it would be things like 98/2 and whatnot?

    Gotcha. Thanks for that...I need to practice a lot more.
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