Significant figures and order of operations

[/0]

According to wikipedia,
"For multiplication and division, the result should have as many significant figures as the measured number with the smallest number of significant figures.
For addition and subtraction, the result should have as many decimal places as the measured number with the smallest number of decimal places."

So what do you do if you have something like 0.235(16.0303+23.5+9.059)? Do you take the result in the brackets to 1 decimal place, multiply by 0.235 then take the answer to the lowest amount of significant figures?

[shinny]

Significant figures is only taken into account at the END of a calculation. So just multiply everything in there as you normally would, and take 3 significant figures.

[/0]

Ooh that simple? Awesome, thanks shinny

And congratulations on your vce results

[/0]

What about in the middle of a question? For example:

You are given mol, and the molar masses of Na, O, H are 23.0, 16.0, 1.00 respectively. You have to find .

Do you have to write or is acceptable?

[shinny]

The second one is acceptable. They don't mark working out for significant figures; only the end result.

[cara.mel]

For your example, your answer is correct.
I am fairly sure you do not need to know this. Only just to make your final answer to the one with the least amount of sig figs (and also to ignore the amount of sig figs in the molar mass even if it is the least accurate thing)

[/0]

What do you do if the question asks you to use 10 tonnes of an element in a question?

10 tonnes = 10^7 grams, but how accurate would this actually be?

[cara.mel]

It's one sig fig. Trailing zeros don't count unless there is a decimal point.

This is because as you put it, 10 tonnes = 10,000,000g. You can not tell me that 10,000,000g has 8 sig figs in it, same deal to any other unit conversion (it would only have more than one sig fig if it was like 10.0 tonnes, in which case it is 3 sig figs)


Also that is what I could call a dodgy question and the author probably meant it to be 2 sig figs, I've never given an answer to 1 sig fig

[/0]

lol thanks caramel, yeah it is a weird case

[Collin Li]

Yeah, prepare to be frustrated throughout the year, as many questions do not have significant figures in mind, even though you're told again and again to pay attention to them!

[orsel]

IIRC:

In VCAA exams, you only have to give the correct s.f. for 1 particular question where they'll say something along the lines of 'giving the final answer to the correct number of significant figures'. For the rest of the questions, you only have to give the answer to +/-1 digit of the 'correct' s.f.

Though you should probably take this with a boulder of salt.

[vcestar]

Another tip is, when you write down your answer, (say 21.3% for eg) write down (3 significant figures) next to it so atleast the examiner marking it knows you didnt forget the significant figure part. So even if the question migh subtly ask for 4 sig figures for example, the examiner could still give you a mark for atleast trying to exercise the significant figure part of the equation.

[cara.mel]

IIRC:

In VCAA exams, you only have to give the correct s.f. for 1 particular question where they'll say something along the lines of 'giving the final answer to the correct number of significant figures'. For the rest of the questions, you only have to give the answer to +/-1 digit of the 'correct' s.f.

Though you should probably take this with a boulder of salt.

That post is mainly accurate.
They will mark sig figs on one question
They will mark states on one question (s,l,g,aq).
 These are not indicated in the question stem which question they will be picking on for marking sig figs or states.

For every other question, you need to include sig figs/states but if they're not super accurate it's no big deal. In other words you need to be sensible and not give you answer like 3.253353456 because 'oh sig figs may not be marked here ^__^'

[/0]

soz another question

How many decimal places do you need to put molar masses to? In one of the solutions they have :/

[orsel]

Use the relative atomic mass given in the chem booklet(from the exam).

Either that, or if its a question that requires you to calculate the molar mass, then just use the appropriate number of s.f.