Unit 3 Questions MEGATHREAD :)

[luken93]

I hate company exams, period.

Couldn't agree more!!
Do you know if VCAA would take the number of significant figures of the molecular masses into account??

I doubt they'd be that nice haha

Was it dividing/multiplying or add/minus? Just making sure it was sig figs and not decimal places...

[nacho]

Wasn't there a post somewhere giving the rules of sig figs?
I just want to make sure i don't lose any marks because of sig figs.

[luken93]

Wasn't there a post somewhere giving the rules of sig figs?
I just want to make sure i don't lose any marks because of sig figs.

If multiplying/dividing, round to the least number of sig figs
2.01 x 3.0 = 6.0

If adding/subtracting, round to the least number of decimal places
245.1 + 20.001 = 265.1

Zeroes placed before other digits are not significant
0.0023 = 2 sig figs

Zeroes placed between other digits are significant
0.0203 = 3 sig figs

Zeroes placed after other digits are significant.

0.00230 = 3 sig figs

[nacho]

How about the rules for what number of sig figs to use, when presented with information from the question.
And, also..
If we are working out, do we need to give answers in sig figs?
Eg, a certain question asks for the mol of element X when we know that:

A + 2B ---> 3AB

And the question says, molar mass of A= 1.001, B = 3.50
and that 5 grams of B completely reacted with excess 'A'.

would i go:
n(AB) = n2/3(B)
n(B) = 3.5/5 = ....                            <---- does this have to be in sig figs?
therefore n(AB) = ....                        <---- or is it only this answer which is in sig figs?

tyvm

[pi]

How about the rules for what number of sig figs to use, when presented with information from the question.
And, also..
If we are working out, do we need to give answers in sig figs?
Eg, a certain question asks for the mol of element X when we know that:

A + 2B ---> 3AB

And the question says, molar mass of A= 1.001, B = 3.50
and that 5 grams of B completely reacted with excess 'A'.

would i go:
n(AB) = n2/3(B)
n(B) = 3.5/5 = ....                            <---- does this have to be in sig figs?
therefore n(AB) = ....                        <---- or is it only this answer which is in sig figs?

tyvm

Just the final answer is appropriate. Although, I like to maintain sig figs through steps, however it is not marked, just the answer.

[luken93]

What about a 2/3 mark stoich question, where you have to say determine the mol of an unknown substance, and then determine concentration or something? Do you have to have the appropriate sig figs for each "significant even" if you like?

Also, your final answer is to the sig figs of your starting data only ie what is contained in the question itself?

[nacho]

dammit why do they even have to make us put sig figs.
why not good old 'round to two d.p'
so much damn easier. sig figs are unnecessarily confusing..

[Graphite]

If you want just write like as many numbers as you feel comfortable then next line write sig figs
Its good in case you stuff up, you don't have to do something again

[VivaTequila]

If you want just write like as many numbers as you feel comfortable then next line write sig figs
Its good in case you stuff up, you don't have to do something again

When writing my answers, I always include at least 5 sig figures, and show these in my working. I get to a final answer and write =3.3913g for instance, and then directly under it, I always without fail write =3.39g w/ sig fig.

Hasn't failed me yet and it's been endorsed by everyone I've asked. Always circle the final answer though if you use this method.

[EvangelionZeta]

Yeah, I'd advise that you keep a decent number of significant figures in your working, but make sure you have the correct number in your final answer. There's usually at least one question on each VCAA examination which requires a solid grasp of significant figures, and anyway, once you hit university, you could find yourself with pretty strict lab markers/tutors.

[luken93]

In Mass Spec, if you had say Ethanol, which H atom would be more readily be lost upon fragmentation to form a peak at m/z 45?

Would the H come off the O, since it next to a more EN atom in oxygen, or is it just as likely to come off any other carbon atom?

[b^3]

Yes the H that is connected to the O is the mostly likely to come off because its the easiest part to slice off.

[thatricksta]

Cheers, the question was to show the formation of the m/z = 15 fragment from the parent ion (ethanoic acid)

The answer said:

CH3COOH+  -->   CH3+ + COOH

Whereas I wrote:
CH3COOH + e-    -->   CH3+ + COOH + 2e-

Is mine necessarily wrong then? (probably hahah)

Both are correct and will get you full marks (apparently this has been confirmed by the chief assessor) according to TSFX


I lurked around on this post (hopefully I didn't miss my answer) and was wondering if VCAA significant figures will take into account the molecular masses. It has been noted that most of the elements in our data booklet are rounded to 3 sig figures, so if we had a question (obviously not this easy) that involved us simply finding the mol of atoms in a particular mass that was weighed to be 4 sig figures, 2.514g it was something like... Calcium which has a molar mass of 40.1, would be then round our answer to 3 sig figures? (0.0627) or 4 sig figures? (0.06269)
Thanks

[luken93]

Yep, still 3 sig figs, as molar mass is still a measurement that comes with significant figures.

[thatricksta]

Yep, still 3 sig figs, as molar mass is still a measurement that comes with significant figures.

Thanks.

My chemistry teacher still thinks it might be 4 sig figures as you should only take uncertainties within the question... I wonder if VCAA will actually treat us like that though... I have never actually had to make the decision in a practice exam