ATAR Notes: Forum
VCE Stuff => VCE Science => VCE Mathematics/Science/Technology => VCE Subjects + Help => VCE Chemistry => Topic started by: Christiano on February 05, 2011, 12:52:38 pm
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My answers for questions regarding molar masses, etc are sometimes slightly inaccurate to what it is at the back of the book - for example, 1.5g/123g/mol gives me 0.012195121, but I round it off to 3 significant figures - 0.0122. However the BoB says its 0.0123. Why is this so?
And accurate do the answers on the exam have to be? For example rounding 0.12349 to 4 sig figures as 0.1234 (instead of 0.1235), is it still considered correct? What is the margin for error regarding this?
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Yeah I was wondering this too. My teacher did say there is a range of answers the examiners find acceptable but I can't guarantee that she is correct for some reason LOL. As for the rounding of 0.12349, I would THINK that rounding it to 0.1234 would be incorrect... but I need clarification on this too
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hey, don't worry about this. Your answers are slightly different because i assume your using a periodic table which gives the molar mass to a different amount of decimal places, than what the VCAA Databook does (all textbooks use the vcaa databook for answers as far as i know)
And as you sadi with the sig figures, it would most likely be 0.1235, as that is the conventional method. I don't know what the margin of error is, but i assume a lot of chemistry students lose their marks here, so just be careful and maybe get used to the databook so you can familiarise yourself with common Mr.
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As nacho said, VCAA's answers come from data in the data book; I would use the databook to do my questions, rather than the periodic table on the textbook. I think VCAA accepts two answers; the first one assuming that you used the rounded answer to each part of the question, and the second assuming you kept the number in the calculator the whole way through the calculation.
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As nacho said, VCAA's answers come from data in the data book; I would use the databook to do my questions, rather than the periodic table on the textbook. I think VCAA accepts two answers; the first one assuming that you used the rounded answer to each part of the question, and the second assuming you kept the number in the calculator the whole way through the calculation.
But to save the hassle throughout the exam, say for example you were doing a titeration Q and you had the 3 steps of finding concentration, moles and concentration, should we round off to a few sig figs but keep the answer in our calc?
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Yup. I also write what's on the calc to a few more sig figs on the side, just in case i have to erase the number on calc
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This is interesting because my highly-esteemed Chem teacher informed us to use the significant figures from the line above and not use the calculator values. Her reasoning being that as Chemists, we deal with measurements not numbers and as such you cannot use a falsely accurate measurement in further calculations. I know in mathematics this is not the case, so it kinda screwed with me a bit...
I shall have to inquire further into this...hmm
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One of our highly esteemed chem teacher says not to round until the end; I think it's more a matter of opinion than anything - probably argued between chemists. That's why VCAA accepts both methods.
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I've been using the data booklet from the start :-\ Although there is no difference between the molar masses on the data booklet and the periodic table at the back of my Heinemann textbook..
I see the logic in how VCAA assesses the answers, but I think i'd find it really, really difficult to be spot on. My answers are sometimes off by a minimal margin due to disregarding numbers beyond the required amount of sig figs. In class, there's always a discrepancy between answers .. especially for those that involve a lot of numbers and sig figs ..
What's the best way to approach this? I need some consistency..
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One of our highly esteemed chem teacher says not to round until the end; I think it's more a matter of opinion than anything - probably argued between chemists. That's why VCAA accepts both methods.
Oh so both are accepted...at least we don't have to worry about that then. coolies
Just curious...does it say anywhere that VCAA accepts both methods? Or is it just a generally accepted thing...
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I've been using the data booklet from the start :-\ Although there is no difference between the molar masses on the data booklet and the periodic table at the back of my Heinemann textbook..
I see the logic in how VCAA assesses the answers, but I think i'd find it really, really difficult to be spot on. My answers are sometimes off by a minimal margin due to disregarding numbers beyond the required amount of sig figs. In class, there's always a discrepancy between answers .. especially for those that involve a lot of numbers and sig figs ..
What's the best way to approach this? I need some consistency..
To be honest, it would seem that if you've shown the use of sig figs, use of consistent calculations (I would advise against using significant figures from above as it is LESS accurate) and rounded correctly, you should be fine. They could see from your workings that you know how to do the questions and you've applied the correct conventions in presenting your answer. Otherwise, there wouldn't be many people that would lose any less than 5 marks!
My advice is to not worry about this stuff, except for sig figures - get used to that.
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Yeah I wish they had someone you can contact about these sort of things!
I think I like thushan's the best, you know that way that you wont have any problems rounding off until the end.
On a side note, should we answer in the following way:
Eg, calculate the mass of 2 moles of Water, correct to 3 sig figs
... = 36g
= 36.0g ?
by this I mean write the full answer first to show the examiner what you've actually calculated, then round it off?
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Yeah I wish they had someone you can contact about these sort of things!
I think I like thushan's the best, you know that way that you wont have any problems rounding off until the end.
On a side note, should we answer in the following way:
Eg, calculate the mass of 2 moles of Water, correct to 3 sig figs
... = 36g
= 36.0g ?
by this I mean write the full answer first to show the examiner what you've actually calculated, then round it off?
I dont think it really matters.
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Yeah I wish they had someone you can contact about these sort of things!
I think I like thushan's the best, you know that way that you wont have any problems rounding off until the end.
On a side note, should we answer in the following way:
Eg, calculate the mass of 2 moles of Water, correct to 3 sig figs
... = 36g
= 36.0g ?
by this I mean write the full answer first to show the examiner what you've actually calculated, then round it off?
Yes, that's correct, assuming 36.0 is 3 signifcant figures (there are some rules, but i dont remember them)
You can check here:
http://www.youtube.com/watch?v=5UjwJ9PIUvE