How do u work out % errors!?

Hii!
I'm really stuck on how to work out percentage errors in chemistry!! I understand that with a measuring cylinder its 0.5 of the smallest graduation divided by total volume, so it would be 0.5/50 for example
But with a burette, you take two readings, so do you multiply your percentage error by 2? And how do you measure the % error of a pipette? Because that doesnt really have a smallest graduation!

Sorry I'm all confused!!! Any help would be great! Thanks :smile:

Reply 1

KP Nuts

I may be wrong, but this is how understand it;

You read burettes to the nearest 0.5cm^3. Therefore if you have two readings of say - initial=2.5cm^3 and final=22.5cm^3. Since each of these have been rounded, the initial could be as low as 2.25 and the final could be 22.75. Therefore, the titre could be as large as 22.75-2.25= 20.5cm^3.

The titre that you will have calculated for use in calculations will be 20cm^3. Therefore, 20.5-20 = 0.5 - this is the maximum error caused by rounding.

(0.5 / 20) x 100 = is the percentage error (i.e. divide by the actual titre and multiply by 100)

If this is incorrect, please tell me

Reply 2

Fluent in Lies

Percentage error is useful as it shows the error in relation to the ammount used (also relative error). 1cm3 is a large error for 2cm measurement, but not so bad for a measurement of 1000cm3.

percentage error = [ (absolute error)/(calculated value) ] x 100.

Where absolute error is the possible range the value recorded could take. It is half the smallest measurable increment used. As explaind correctly by the person above. On a scale 1 | 2 | 3 | 4 | 5. A reading of 4 could mean anything from 3.5-4.5. But you could only effectively measure it to 4. so the abs error is 0.5.

Reply 3

KP Nuts

so is mine correct, because the measurable increment on a burette is 0.5. Therefore percentage error would be (0.25 / 20) x 100? Or, as the first person said, do u multiply by 0.25 by 2 since you are taking 2 measurements (I didnt understand what they meant by this at first)

Reply 4

Fluent in Lies

For a burette you are taking two readings yes. say the liquid is at 41.50cm3 and runs down to 26.30cm3 those could have been:
41.55 - 26.25 = 15.30
or
41.45 - 26.35 = 15.10

So absolute error, or 'estimated uncertainty' is added.

percent error = (1/15.2) * 100

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For a pipette, I asume you mean a volumetric pipette and not a graduated or dropping, the abs error should be on the flask, or if not your teacher should know.

Reply 5

wizz05

we were told that the % errors were

0.5cm3 for the measuring cylinder
0.05cm3 for the 25cm3 pipette
0.15cm3 for the 250cm3 measuring cylinder
and 0.15cm3 for the burette (as you measure at the top and the bottom and also there is always a drop on the end)

hope this helps