How to keep proper significant figures in CIE A level chemistry test?
Hello everyone,
I have a very big question
ow to keep proper significant figures in stuctured question calculation? Because everytime I check the mark scheme, my answer has more decimal places or less s.f.s than the standard answer. So what do you think is the key to keep proper significat figures? I am really worried, because there're only 13 days before the paper 2 test begins.
Thank you
I have a very big question
ow to keep proper significant figures in stuctured question calculation? Because everytime I check the mark scheme, my answer has more decimal places or less s.f.s than the standard answer. So what do you think is the key to keep proper significat figures? I am really worried, because there're only 13 days before the paper 2 test begins.Thank you
Original post by Borek
Can you give an example of calculations where your result differs from the mark scheme?My bet is that you are treating addition/subtraction incorrectly, but that's just a random guess.
IMHO mark scheme is wrong. Should be 9.70x10-3, and the mass should be reported to 3 sig figs (1.28 g). However, all calculations should be done with so called guard digits (ie copy numbers for calculations using several additional digits, say 2 or 3).
In general: report as many sig figs as the data support, but calculate with full precision available.
In general: report as many sig figs as the data support, but calculate with full precision available.
Original post by Borek
IMHO mark scheme is wrong. Should be 9.70x10-3, and the mass should be reported to 3 sig figs (1.28 g). However, all calculations should be done with so called guard digits (ie copy numbers for calculations using several additional digits, say 2 or 3).
In general: report as many sig figs as the data support, but calculate with full precision available.
In general: report as many sig figs as the data support, but calculate with full precision available.
What do you mean by guard digits?
When you copy calculated result to use it in further calculations, write several additional digits.
Say, you are need to calculate 1.234 times 5.678. 4sd in both numbers, so you report the result as 7.007. But for your own use copy the result as 7.00665 and use this value in further calculations. Last digits (65) are guard digits - they are there just to keep the accuracy, you never report them.
Say, you are need to calculate 1.234 times 5.678. 4sd in both numbers, so you report the result as 7.007. But for your own use copy the result as 7.00665 and use this value in further calculations. Last digits (65) are guard digits - they are there just to keep the accuracy, you never report them.
Original post by Borek
When you copy calculated result to use it in further calculations, write several additional digits.
Say, you are need to calculate 1.234 times 5.678. 4sd in both numbers, so you report the result as 7.007. But for your own use copy the result as 7.00665 and use this value in further calculations. Last digits (65) are guard digits - they are there just to keep the accuracy, you never report them.
Say, you are need to calculate 1.234 times 5.678. 4sd in both numbers, so you report the result as 7.007. But for your own use copy the result as 7.00665 and use this value in further calculations. Last digits (65) are guard digits - they are there just to keep the accuracy, you never report them.
Thank you.What if there's 3dp times 4 dp , e.g. 1.112x8.31?
BTW, will the examiner really want to deduct all marks in a question if I put some wrong s.f. in every step of calculation?
Original post by jamesjiang2015
Thank you.What if there's 3dp times 4 dp , e.g. 1.112x8.31?
You should report the result as 9.24, but if the number is required for later calculations, you should note it down and use at least 9.2407.
Imagine you are asked to calculate 2.0*0.57. That yields 1.14, you have to report it using 2sf, so 1.1.
Now you are asked to calculate 18*0.13. That yields 2.34, reported as 2.3.
Now you are asked about the sum of these two products.
Naive way of calculation: 1.1+2.3=3.4... but this is wrong. 2.0*0.57+18*0.13=3.48, to 2 sf it is 3.5, not 3.4.
Why the difference? Because you rounded to early, You should add not 1.1+2.3, but 1.14+2.34 and round down the sum. 4 would be your guard digit (it happens to be identical for both numbers in this case).
BTW, will the examiner really want to deduct all marks in a question if I put some wrong s.f. in every step of calculation?
No idea, but why take a risk?
Well , the test is very very very very very very very very very very very very very very very very very very very important , so I don't think it is a good idea to take a risk.
Thank you very much for your help!
Thank you very much for your help!
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