Percentage uncertainties | A level Chemistry

Percentage Uncertainty
(Percentage error)
Required Practical
PAGs.
A level Chemistry
Question Download:
drive.google.com/file/d/13pmd...
00:00 Introduction
01:01 What is Uncertainty?
02:13 Reading or Measurement?
05:56 Measuring a Change
08:43 Uncertainty in Repeats
09:51 Percentage Uncertainty
12:46 Total % Uncertainty
14:02 Reducing % Uncertainty
16:08 Using % Uncertainty
18:01 Example questions 1 and 2
20:04 Example questions 3 and 4
21:19 Example questions 5 and 6

[@meggiedoeshair]

Awesome! These questions always get me stumped 😬

[@chemistrytutor]

They're tough, but they can be quite similar. Lots of practice is key 😃

[@johnsungunlee]

thank you so much for this! I was confused as to when you times the uncertainty by 2; I thought you had to times it by 2 every single time. Now I've realised you have to times by 2 when you take two readings e.g a temperature change.

[@chemistrytutor]

Excellent 😀
Although, I should say about the burette, that normally gets adjusted for you. You are usually given 0.15 and this has been tripled to account for reading 1 and reading 2 AND the judgement of the endpoint/drop size. So you don't normally have to do any multiplying for burette uncertainty

[@johnsungunlee]

@@chemistrytutor ohh ok thanks sir🙏

[@rachaelkenyon4712]

thanks! Finally understand this now!

[@chemistrytutor]

Lovely! Thanks for the feedback 😀

[@Masowe.]

great way to learn. testung knowledge straight away, thank you

[@chemistrytutor]

You're welcome! Thanks for the positive feedback 😀

[@user-tb7no8fd8p]

Thank you very much.

[@chemistrytutor]

😀 glad it's useful!

[@donia-a966]

would the last part of Q6 about calculating percentage uncertainty of burette, don't we have to multiply the uncertainty by 2 as it is a measurement so instead of 0.15 It would be 0.15 *2=0.3?

[@chemistrytutor]

This is good thinking - but burettes is actually an exception. The uncertainty of each reading is actually 0.05 and then doubled = 0.1. We then add a third 0.05 as the drop size and the end point judgement can be inconsistent. *So* the multiples have been done for us when we arrive at 0.15

[@phildavies4814]

Many thanks for the video. In Q5 (21:49) option A is about "25cm3 measured with a burette with an uncertainty of +-0.1cm3". I doubled this because you have to measure twice when using a burette (similar to the temperature change in D). So I got % uncertainty of 0.8% for A, so answered C which is lower at 0.67%. But you didn't double the burette uncertainty, presumably taking this as the *total* uncertainty for both measurements on the burette. And the correct answer is then A at 0.4%, so I got it wrong. But the question doesn't actually say whether the stated uncertainty for the burette is for a single reading or a total (whereas question 6 clearly does: "These errors take into account multiple measurements"). Is there some way to tell whether the stated uncertainty is for a single reading or both, if the question doesn't say?

[@chemistrytutor]

Yes, burettes are the odd one out. The uncertainty given *always* includes any repeats. The total uncertainty for a burette is taken as +/- 0.15 and that is what you are expected to use it as, and not change the value 😀

[@phildavies4814]

Thanks very much for the quick reply. I will try to remember this fact :-)

[@chemistrytutor]

@@phildavies4814 😀