The Student Room Group
Reply 1
Yeah, I'm really confused about this too. Basically, I think you're meant to double the % error when you weigh it twice....
Reply 2
no it doesnt say how many times it was weighed. This question was from a june 05 edexcel 3b paper.
Reply 3
i was going to post exactly the same question as you as i came across it today. I personally think that this is an error because it says...The balance used to weigh the mag carbonate is accurate to 0.01g. Similar questions have appeared in other papers and i used the same method (divide the mass of element and divide it with the balance given e.g 0.01) to answer this one...i would have also done 0.01/ 0.21. It doesn't indicate anywhere that it should be doubled. :confused:
In the June 2006 Edexcel 3B, you had calculated the molar mass of a substance as 115 (You did not know what the substance is).
http://www.sendspace.com/file/vc5crf (2nd page).
Uncertainty in the reading of molar mass for +- 0.91%.
Reply 5
I think that its 0.02 because you have to effectively take two readings - the initial reading without the mass (probably 0.00) and the reading with the mass on it - and both readings have an error of 0.01
Reply 6
it's 0.02 coz:
0.21 - 0.01 = 0.20
0.21 + 0.01 = 0.22
0.22 - 0.20 = 0.02
(0.02/0.21) * 100 = 9.5%
hope tht helps :smile:

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