Back titration calculations (for aspirin investigation)

    • Thread Starter

    Hi I'm really suck, and would be extremely grateful if anyone could help me. I did a back titration to test the purity of aspirin. I did this with pure aspirin, so it should be pretty close to 100% pure. I started with 1.5g of apsirin, but the results of my calculations suggest I started with about 2g of aspirin, which is obv a load of rubbish. Can anyone see where I'm going wrong?
    I hydrolysed the aspirin with 25cm3 of 1.0M sodium hydroxide, and then titrated this solution against 0.1M hydrochloric acid. My average titre is 7.7cm3. Is there something wrong with my method maybe? Or the calculations? I'm really stuck and I won't have much chance to talk to my teachers before my investigation is due in.

    Here are my calculations:

    Average titre = 7.77cm3 = 7.77 x 10-3 dm3
    conc HCl = 0.1M
    total moles NaOH added = 0.025dm3 x 1.0M = 0.025mol
    mol HCl = mol excess NaOH
    mols HCl = conc x volume = 0.1M x (7.77 x 10-3)
    = 7.77 x 10-4 mol NaOH excess

    moles of NaOH at start: moles = concentration x volume
    = 1.0 x 0.025 = 0.025 mol

    initial moles NaOH - excess NaOH = NaOH used in hydroylsis
    0.025 - (7.77 x 10-4) = 0.024 moles NaOH used in hydrolysis

    then I dived this by two, becuase in the reaction equation there are two molecules of NaOH for every aspirin

    which gives 0.012

    then mass = moles x Mr = 0.012 x 180.2 = 2.18 g

    This can then be used to work out the percentage purity, but as I only started with 1.5g of aspirin and this claims I started with 2.18g, there isn't readlly much point in % purity until I get this bit right.

    I think I must be doing something stupid to be getting such silly results.

    Thanks sooo much to anyone who even bothers to read this, and any help would be great, and obv I will rep even though mines not worth much. Thanks! :confused:

    Well, I've spent a good 10 mins looking at this problem now, and the calculation seems correct. That is extremely queer.

    The only things I can see that may have possibly gone wrong are forgetting to reset the balance to 0 before weighing your aspirin out, leading to a mass you think is 1.5g but which may have been more. You might have been using contaminated acid causing a wrong titre volume to be given, by not reacting as intended. Lastly perhaps misjudging the endpoint of the reaction slightly? If you used the same indicators we used when I was at school then it changed over a range, and the endpoint could easily be missed.

    If you do have a chance to run through the practical one last time before you hand it in then just check everything again. Or else just adapt your results to show what you want them to... Not that I'm condoning cheating of course, but you do need the A's to get into medschool :rolleyes:

    Good Luck!
    • Thread Starter

    Thanks so much for your help. This was actually the third time I did it, the first time I got an av titre of ~ 4 :confused: . I showed everything to my teacher and she made me repeat it. I still got ~4. SO she got me some new HCl, and suddenly the average titre was up to 7. I thought that had fixed my problem, but obviously not. I don't have time to do it again, so i guess I might have to change the results slightly . Thanks so much for your help though, at least I know its not the calculations where I'm going wrong, although tbh it would make things a lot easier if it was. This whole experiment has been a complete disaster.

    Thanks again


    It was so long ago that you posted this but I'm just going to try my luck anyway.... you dont happen to rememmber where you got your method for the back titration of aspirin? I can't seem to find a method anywhere

    (Original post by Nahya)
    It was so long ago that you posted this but I'm just going to try my luck anyway.... you dont happen to rememmber where you got your method for the back titration of aspirin? I can't seem to find a method anywhere
    Mrs_Bellamy has probably by now left school, graduated, got married and is currently living in Madagascar researching lemurs with her husband and four kids ...

    ... probably.
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