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Homework Help: Aspirin Titration lab

  1. Jul 20, 2017 #1
    Really struggling with this!
    Here is the data:
    Concentration of NaOH: .104M

    Mass of Aspirin #1: .367g

    Mass of Aspirin #2: .365g

    Volume of NaOH mixed with Tablet 1: 50ml

    Volume of NaOH mixed with Tablet 2: 50 ml

    Trial 1

    Mass of Na2CO3: .065g

    Initial volume of HCl: 18.80mL

    Final Volume: 29.50 mL

    Trial 2

    Mass of Na2CO3: .100g

    Initial Volume: 29.50mL

    Final Volume: 39.40mL

    Titration procedure:

    1.) refill the burette with the HCl solution

    2.) add a few drops of phenol red to the flasks with aspirin.

    3.) titrate your aspirin in the flasks with the HCl solution.

    Titration for trial #1 initial V: 2.80mL

    Titration for trial #1 final V: 18.40mL

    Titration for trial #2 initial V: 18.40mL

    Titration for trial #2 final V: 33.65mL

    Concentration of HCl for T1: .112M

    concentration of HCl for T2: .189M

    Average: .150M

    1. Using your data, calculate the moles and mass of acetylsalicylic acid in each tablet.

    Any help would be appreciated!!
  2. jcsd
  3. Jul 21, 2017 #2


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    I think it is us rather who need help with this problem set out without any narrative.

    I'm guessing that one or more HCl Solutions were first standardised by titrating against Na2CO3 solutions
    This is a fairly elementary and almost starting point in volumetric analysis, I presume you can calculate the concentrations of these HCl solution(s).

    . Harder to see the sense of the rest of the problem.

    Aspirin is acetyl salicylate. I can justify dissolving it in NaOH, because concentration would be about twice the solubility in water of acetyl salicylate.

    Save error on my part the molarity of the NaOH is about 20 times that of the aspirin, which would limit the accuracy of any titration, but you could still get a moderately accurate estimation.

    It took me a little time to puzzle out. I don't think I have ever come across a titration using this principle.

    What I think they are doing is: the OH- is subtracting protons from the acetyl salicylate:

    Hacsal + OH- → acsal- + H2O

    Or if you like the acetyl salicylateIs subtracting OH- from the solution, leaving less OH- than there initially were in the NaOH.

    When you then titrate this mixture with HCl, you titrate only these leftover OH- and not those that have been subtracted by the acetyl salicylate - because the phenolpthalein changes colour by the time you have descended to pH 8, but at that pH salicylate is still in the form acsal- (its pKa is about 3.5). Hope that helps
    Last edited: Jul 21, 2017
  4. Jul 22, 2017 #3


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    However I am not convinced that to titrate aspirin it is necessary to do it this way.

    Even if the acid is not completely soluble, as you add NaOH it should dissolve, and should all have dissolved as you reach the colour change end point. You might have to wait a little if these are actually aspirin pills.

    Could you reproduce the original question verbatim?
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