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    In our titration analysis we found the total percentage uncertainty was 1.05%. Unfortunately, my result was off by 4.2% (I had 0.0958, the correct concentration was 0.100) which seems quite worrying. Is this anything to worry about? This was the first titration I did, and I think the biggest error was using the pipette, the NaOH was bouncing up and down when I moved the pipette but I couldn't see any air bubbles. I would actually quite like to do it again just to get more experience, but I am a bit concerned that my result is so far off what it should've been.
    I'm a year 12 student.
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    (Original post by ralph9694)
    In our titration analysis we found the total percentage uncertainty was 1.05%. Unfortunately, my result was off by 4.2% (I had 0.0958, the correct concentration was 0.100) which seems quite worrying. Is this anything to worry about? This was the first titration I did, and I think the biggest error was using the pipette, the NaOH was bouncing up and down when I moved the pipette but I couldn't see any air bubbles. I would actually quite like to do it again just to get more experience, but I am a bit concerned that my result is so far off what it should've been.
    I'm a year 12 student.
    Instrumental error is a form of random uncertainty, which assumes that you use the apparatus to the absolute limit of your possibilities. This is, of course, highly unlikely.

    A 5% deviation from the actual value is quite reasonable.
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    Thanks church, that's very reassuring.
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    (Original post by ralph9694)
    Thanks church, that's very reassuring.
    church?

    you should stay off the wacky baccy ...
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    (Original post by ralph9694)
    In our titration analysis we found the total percentage uncertainty was 1.05%. Unfortunately, my result was off by 4.2% (I had 0.0958, the correct concentration was 0.100) which seems quite worrying. Is this anything to worry about? This was the first titration I did, and I think the biggest error was using the pipette, the NaOH was bouncing up and down when I moved the pipette but I couldn't see any air bubbles. I would actually quite like to do it again just to get more experience, but I am a bit concerned that my result is so far off what it should've been.
    I'm a year 12 student.
    It's just a school experiment. You're definitely within the range of experimental error allowing for things like air bubbles, the pipette (I found that very fiddly as well), uncertainty as to where the end point is (depends what indicator you used - we used methyl orange which isn't as clear-cut as phenolphthalein) and possible error when making up the standard solution.
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    (Original post by charco)
    Instrumental error is a form of random uncertainty, which assumes that you use the apparatus to the absolute limit of your possibilities. This is, of course, highly unlikely.

    A 5% deviation from the actual value is quite reasonable.
    5% is nothing. We got people who found that the atomic mass of magnesium was 18 in our experiments. The methods in the PAGs (compulsory practicals) usually aren't perfect so that the questions that come with them have something to ask about.
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    (Original post by charco)
    church?

    you should stay off the wacky baccy ...
    whoops, spell check strikes again.

    We used phenolphthalein, when it neared the end I did it a drop at a time and it was pretty obvious when it changed.
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    (Original post by ralph9694)
    whoops, spell check strikes again.

    We used phenolphthalein, when it neared the end I did it a drop at a time and it was pretty obvious when it changed.
    Don't forget that there are several steps in a titration.

    Instrumental inaccuracy
    You have the inaccuracy of making the solutions:
    1. weighing the solute
    2. Making up to the mark with a volumetric flask

    3. You have the inaccuracy of transferring a 25ml aliquot into a conical flask
    4. You have two readings on the burette

    Procedural (systematic) inaccuracy
    Gauging the endpoint
    Not rinsing the pipette and burette with the solutions to be used

    Typical values for instrumental easily add up to 4-5%

    For example
    1. To make 0.1M NaOH you have to weigh out 1g. If your balance is 2dp then the inaccuracy will be 1.00 ± 0.01 = 1%
    2. The 250ml volumetric has (say) an inaccuracy of ± 0.4 ml = 250 ± 0.4 = 0.16%
    3. Pipette has an inaccuracy of (say) ± 0.14 = 25 ± 0.14 = 0.56%
    4. Burette has an inaccuracy of ±0.05, but you have to take two readings = 25 ±
    0.1 = 0.4%
    Total instrumental inaccuracy = ± 2.12%

    If you manipulate everything perfectly ...
 
 
 
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