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Homework Help: Need to find the equivalence point volume from the first derivative graph?

  1. Jul 10, 2011 #1
    1. The problem statement, all variables and given/known data

    This was done in a lab, we titrated 0.2M NaOH with an unknown concentration of acetic acid.
    I've used excel to graph my first derivative of the data.

    2. Relevant equations
    (change in pH)/(Change in volume)

    3. The attempt at a solution
    My professor talked about drawing two trend lines, one from the left and the other from the right, and wherever they intersected, would be the equivalence volume. I'm just wondering if My trend lines should go through those two points at the top? I remember him saying not to force the lines to intersect at the highest point. But isn't it true that the equivalence point will have the largest slope?

    Anyway, to me, the equivalence point volume looks to be around 19.9 mL

    Did I do this right?

    Attached Files:

    Last edited: Jul 10, 2011
  2. jcsd
  3. Jul 11, 2011 #2
    In a perfect world, the equivalence point would have the highest slope. But since all your measurements have some degree of inaccuracy (and slope measurements in particular can be very inaccurate), the equivalence point might not correspond exactly to your highest slope point. Don't force your lines to go right through the top points, but don't ignore them. It would be best if you can choose trend lines that come fairly close to all the points.

    I would think somewhat to the right of 19.9, but I understand how you got that. I think you have the right idea.
  4. Jul 11, 2011 #3
    Alright, thank you -- the part about whether to draw them through the top two points was the most confusing. I printed it out and drew the trend lines, and I agree thats its more like 19.97mL.
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