chem questions!!

Hi

We did a titration in our chemistry lesson today, and I got a bit confused about a few points, can anyone help?

When calculating the mean titre do you add up all the ones that agree (that are within 0.1 or whatever) and then just divide by the number you've found the sum of? Do you have to round your answer for the mean titre to the nearest 0.05 because this is how accurate the burette readings were, or can you just write it to 2 decimal places?

If a mass balance is accurate to 0.01g then how do you calculate the percentage error of say 60.02g? Also, if the burette readings are made to the nearest 0.05 how do you calculate the percentage error for these?

Thanks for any help!

Reply 1

Jooeee

Can't exactly remember about 1st one, but i think you can just round to nearest 0.05. But, the percentage error = error/value x 100

i.e for the balance 0.01/69.02g x100

Reply 2

hodgey90

Jooeee

Can't exactly remember about 1st one, but i think you can just round to nearest 0.05. But, the percentage error = error/value x 100

i.e for the balance 0.01/69.02g x100

ditto,

Reply 3

minnie

thanks for your help

so if the burette reading was 55.25 then the percentage error would be 0.05/55.25 x 100?

would it be wrong to not write mean titre to nearest 0.05?

Reply 4

Zygroth

% Error for burettes and anything which requires 2 measurements is twice the normal % error, because you have to take a start reading as well as an end one, and there's an error for both of those.

For the mean titre, I think you need 3 readings within 0.1, and the mean is to the same accuracy as the readings - you can't get more accurate than what you're using.

Reply 5

Dentro

which exam board are you doing?
yes you take the average of the concordant titres (which are the ones within 0.1 range ), and the it depends on what exam board your doing, for OCR you have to round up your answer to the nearest 0.05 (i am not sure about other exam boards)

lol (Jooeee) gave you the correct answer for % error... :yep:

Reply 6

Aaron_1507

minnie

You DO NOT round to 0.05 you use the exact numbers your calculator tells you.

Reply 7

Dentro

it depends on what exam board you are doin, OCR would NOT accept anything that isn't rounded up to 0.05

for the % error of your tittration its 0.05/(your average titre) *100

Reply 8

Zygroth

Dentro

for the % error of your tittration its 0.05/(your average titre) *100

If you can take 2 readings while getting only the error of 1, I'd like to hear how you do it, because I certainly can't.

Reply 9

Dentro

but you don't actually know your error, i mean you don't know if you've got error on one reading and none on the other.

so you still need to take the average titre and follow the same steps

I think

Reply 10

minnie

so if you got 31.05 and 31.10 for you two readings, what would you calculate as your average titre from these?

Reply 11

Zygroth

Dentro

but you don't actually know your error, i mean you don't know if you've got error on one reading and none on the other.

By that logic you have no error when you take 1 reading :rolleyes:

I think 31.08 is okay for the average, though I'm not too sure. It shouldn't be more accurate (to 3dp) than your readings though I'm pretty sure.

Reply 12

Dentro

you always have an error..i might have explained it wrong.

like say you have two readings 31.05 and 31.10 (i just realised that i got the question wrong; never mind ), so your average titre 31.08, and %error is (0.005/31.08)*100

I AM SO SORRY about before, next time ill make sure i read the question proberly