VCAA 2012 Chemistry Unit 3 Exam Discussion

[Shenz0r]

For testing for pure propyl propanoate, I wrote that you could run it through IR Spec and view its spectrum, if it has a trough for the C=O bond and C-H bond but no troughs for O-H (alcohol) and O-H (acid), then it would be deduced that the ester would be pure.

[ezye21]

for question 6. II it said the average Fe content of  each of the vitamin C tablets
 i got the amount in mg for each tablet... not overall ? -.- so i got 3 values due to three different absorptions...

[acinod]

answer 5 a0 i) is only two carbon environments not 3 due to the two methyl groups being the same carbon environment
 otherwise rest looks good
and question about pure i reckon in the first bit you needed to mention fractional distillation and to ensure your product was 100% pure you would run the sample that you got from your fractionall distillation in a HPLC and then run a known standard of propyl propanoate and then if the Retention times are the same and no other peaks are present then the sample is 100% pure

Yep I fixed it up.

But yea you could also do the potassium dichromate colour test for alkanols and to test if theres carboxylic acid you see if it reacts with sodium carbonate and observe for bubbles. But i guess VCAA won't expect most students to get this since it's a bit specific theory and the more general use of a chromatography or spectroscopy should be the more popular approach.

[sahil26]

i think i lost 8-12 marks.... reckon i miggght.... sneak in an A+ ? i know it depends on the cut-off... but someone... please give me hope

[AllAboutTheLGs]

i lost 2 marks definitely, which could rise to 4 depending on the explain questions. -sigh

[Destiny]

Guys, studying for the GAT would be a better use of time than moaning... >;[
Well, I know I stuffed up chem, but who cares...?  Build a bridge and get over it or you're going to screw up another exam.

[sin0001]

Hey for 6) multiple choice, I thought it was B (glycine) because there's like no non-polar ch2 and ch3 chains in the z group?
And being the stoner I am, for the average 6aii) I added 100, 200, 300 and 400, to find the average concentration of Fe, and then I divided that by 4, and I worked from there onwards. Will that get like a conseq. mark or something :S

[starburst.95]

so could you write just fractional distilation for testing if the propyl propanoate is pure ??

[charmanderp]

so could you write just fractional distilation for testing if the propyl propanoate is pure ??

No. Fractional distillation isn't actually a way of testing for purity, it's a method for achieving purity.

[Shenz0r]

so could you write just fractional distilation for testing if the propyl propanoate is pure ??

No.

You would use fractional distillation for collecting - but not for testing to see whether it is pure.

[gggggg1]

could you wright TLC for the purity question?

[Shenz0r]

Don't think so - if you were to perform chromatography on it you would use GLC I think.

[thushan]

could you wright TLC for the purity question?

Don't think so, because I'm not sure whether any of the compounds could absorb UV light.
If they could though, then TLC is completely valid.

[starburst.95]

so could you write just fractional distilation for testing if the propyl propanoate is pure ??

No. Fractional distillation isn't actually a way of testing for purity, it's a method for achieving purity.

but i said that you apply suuficent heat to vaporise the propyl propanoate and if fractions are collected before and after its impure...

[SenriAkane]

i think i lost 8-12 marks.... reckon i miggght.... sneak in an A+ ? i know it depends on the cut-off... but someone... please give me hope

High A or Low A+ is really what you are looking at.