The Student Room Group

Burette Error Question

Hello everyone

I need your help.

Here is a question that I found in a recent past paper:
++++++++++++++++++++++++++++++++++++++++++++
(i) smallest division on burette scale

(ii) estimated error in reading a volume

(iii) estimated maximum error in the titre

(iv)During one titration a student reads the burette twice.
Each reading has an error but the titre has no error. Explain how this can happen.
+++++++++++++++++++++++++++++++++++++++

Well, for (i) I say +/-0.10 cm3
For (ii) the error will be half of this so ans: +/-0.05 cm3
For (iii) the error will be twice this (one for final reading and one for initial) so ans: +/-0.10cm3

Correct so far?

The thing that is making me confused is part (iv). I have absolutely no idea what they are talking about!!!!!!!!!!!!!!!!!

Reading has no error? So does that mean that the titre reading is perfect? How can that be?

Please help me folks.

Thank you for your kindness.

Remember, love is the only reality; everything else is just an illusion. (From a cool book I am reading right now)
If he made the same error each time then both readings have an error but the titre value won't (because the errors are equal so there is the difference between start value and end value is the same)
The two errors cancel each other out so say the first error was +0.03 of the real value and the second error was +0.03 because you take the Final away from the initial the two errors cancel each other out :smile:
Reply 3
Hi friends,
Thanks for the reply.

But me still puzzled.

Take for instance, I do the following calculation:

x - y

and the error in each quantity is +/-a, the the error for the final result will be +/-2a?

This was the same reasoning I applied to my past paper question (iii).

But how can we go from maximum error to no error at all? In what case will the max occur and in what case the error will be eliminated?

The (utterly useless) mark scheme has something about "error had to be in the same direction."

What the peanut butter and jelly sandwich does this mean?

Starting to get hungry.
Reply 4
Well it's actually quite simple. During a titration, you have to read the burette two times: the initial reading (a) and the final reading (b) after titration is complete.

The titre is (b) - (a). Say the experimenter read each reading wrong by +1 unit.

Then the titre is [(b)+1] - [(a)+1] = (b) - (a)
Reply 5
Thanks Edamame:
OOOOOOOOOOOOO

Me thinks I got it.

But see, the thing is, how does one know if he got a positive or negative error.

When I read an instrument I know that my error is +/-0.50 cm3.

But how do I know when a reading has a positive error or a negative error?

I think maximum error occurs when one reading has a positive error and the other reading has a negative error and the errors add up:

[(b)+1] - [(a)-1] = (b) - (a) -2

where the error in (a) is -1.

Am I correct on this?

If I am, then I still wonder, how does one know what error (+/-) occurs in a reading.

I really appreciate TSR's input.

Oh, if anyone needs CIE past papers, feel free to pm me.
Reply 6
Yes, you're right, if the error in the reading were +/- 1, then the maximum error in the titre would be +/- 2. In the question, it just SO HAPPENS that there is no error in the titre, but if you were quoting the result, you would want to write the +/- 2 bit at the end. That there was no error is basically a lucky coincidence.

There's not really a way for you to know which way the errors go in reading off from a burette I'm afraid. That's why you always write the error as (+/-), and this is the reason why you add up (absolutely) the absolute errors when adding/subtracting.
Reply 7
Right!
Thanks Sinuhe.

I know when doing calculations you have to add errors. But I see where you guys are comming from.

sweet
Reply 8
As a general rule (if it helps):

If you add/subtract two values, you sum the errors.
If you multiply/divide two values, you sum the squares of the fractional errors (i.e. the error divided by the value) to get the fractional value in the final answer.
Reply 9
@Kyle_S-C:
Thanks mate!

Good key points. Sweet.
Reply 10
So how to explain the last question ?? iv ??
Reply 11
Im sorry. But i just want to ask. What is the actual answer for the iv question ??