Uncertainties in Lab Work

It may be holiday time in the Northern hemisphere but that doesn’t say things go quiet in the world of blogging. If you are a student who is about to start on your IB Chemistry course, or, if you are a student who has yet to do their IA this post is for you!

When recording measurements or data in lab work it is vital that you record things correctly, especially in your IA (that’s internal assessment to all you new Chemists!).

At IB level, it is now no longer acceptable to record a volume of say, 8 mL – we need to know how certain (or uncertain) you are with regards to this measurement. The accepted way of recording the uncertainty is to look at the apparatus in question, look at the scale and the uncertainty will be half the smallest division.

Using the photos below we have a 10 mL and a 250 mL measuring cylinder (apologies for the bad photos – they are mine).

10 mL cylinder:

10ml measure

10cm meas up close

250mL measuring cylinder:

25cm meas

 

250cm meas up close

If we were to measure, say 9 mL into both measuring cylinder, the uncertainty would be 9.0 mL +/- 0.1mL for the 10 mL measuring cylinder and 9.0ml +/- 0.5 mL in the 250 mL cylinder. If we take a step back and think about this it makes sense – a smaller measuring cylinder will deliver a more certain volume than the larger one.

We can take this a step further and calculate the percentage uncertainty. This will help to calculate an overall uncertainty when multiple measurements (and multiple quantities are being used).

To calculate the percentage uncertainty we use:

[Uncertainty / Measurement ] x 100

So using the above examples, in the 10 mL measuring cylinder the percentage uncertainty is:

[0.1 / 9.0] x 100 = 1% (1 sf)

And the 250 mL cylinder gives:

[0.5 / 9.0] x 100 = 6% (1 sf)

Which further shows how we are less certain of the measurement in the larger container. By the way, it is common practice to quote the percentage uncertainty to 1sf which corresponds to the fewest number of sf in the calculation (ie, 0.1 or 0.5)

If you want a bit more practice at this, here are a couple more to try:

Q

As ever, it would be good to see your results or hear from you if you have any questions regarding this post. Please post your comments below.

2 Comments
  • A
    November 7, 2015

    If the smallest division in the 250 mL graduated cylinder is 2 ml, and uncertainty = smallest division/2, why have you stated it to be +/- 0.5 ml and not +/- 1 mL.

    (2 mL/2 =1)
    Please explain.

    • triplea_da
      December 1, 2015

      Hi amallama9889@hotmail.com,

      You are correct – it is me who has made the mistake!

      Thanks for peer reviewing my article.

      Dave

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