ATAR Notes: Forum

VCE Stuff => VCE Science => VCE Mathematics/Science/Technology => VCE Subjects + Help => VCE Chemistry => Topic started by: Mao on March 30, 2008, 05:40:56 pm

Title: Sig Figs...? =\
Post by: Mao on March 30, 2008, 05:40:56 pm: When doing extended response, and each sub-question asks a result that depends on the previous question, I always seem to deviate a bit at the end from the solutions sheet, because I did not use the result from the previous question with the correct sig-figs, and used the more accurate one on the calculator instead.

Does VCAA do the same thing? Will they take a mark off if I deviate a bit? If so, do I ALWAYS use the values from the previous question?

I thought we were supposed to keep one extra digit until the final answer...
Title: Re: Sig Figs...? =\
Post by: Collin Li on March 30, 2008, 05:46:51 pm: I've noticed that VCAA does similar things (when doing their past papers and comparing to the examiner reports).

But, I always kept the 10 or 11-digit number that the calculator keeps, and I got 45 in Chemistry. I don't think you lose marks for it. I know I lost marks on other things, especially for the Unit 3 exam.
Title: Re: Sig Figs...? =\
Post by: cara.mel on March 30, 2008, 06:02:28 pm: I'm pretty sure they allow for you being a few things off, eg 0.379 vs 0.377.

I always used the big long calculator number too :)
Title: Re: Sig Figs...? =\
Post by: Mao on March 30, 2008, 08:04:23 pm: *phew*

much better :D
Title: Re: Sig Figs...? =\
Post by: /0 on March 30, 2008, 09:07:19 pm: I have a problem with sig figs... best explained with an example.

You store:
$3.52 \times 0.509 = 1.79168$

You write:
$3.52 \times 0.509 = 1.79$

For the final part of the question, you must take the previous answer and multiply it by 4.20:

You store:
$1.79168 \times 4.20 =7.525056$

And so you write, for the final answer:
$1.79 \times 4.20=7.53$

But $1.79 \times 4.20 = 7.52$ to 3 sig figs.!!!

So for the sake of accuracy, you have used the values stored in your calculator, but that means the equations you write down, using the rounded values, are incorrect!

How to remedy this?
Title: Re: Sig Figs...? =\
Post by: /0 on April 01, 2008, 02:25:54 pm: Any ideas? o.O
Title: Re: Sig Figs...? =\
Post by: Mao on April 02, 2008, 06:23:52 pm: well, keep in mind that the numbers arent actually actual values, they are approximations.

if you feel the NEED to be pedantic:

$3.52 \times 0.509 = 1.79168\approx 1.79$

$\sim 1.79 \times 4.20 \approx 7.53$

=S

but there really is no need
Title: Re: Sig Figs...? =\
Post by: /0 on April 02, 2008, 06:34:07 pm: Lol, thanks Mao, I'm just worried about pedantic examiners
Title: Re: Sig Figs...? =\
Post by: bec on June 07, 2008, 05:41:49 pm: Do we take periodic table values into account?
eg. the molar mass of hydrogen is 1.0 in the data booklet, so if a question involves values that are all to 4 sig fig, and during your calculations you work out the molar mass of something with hydrogen in it, should your final answer be to 2 sig fig or 4?