2009 Chemistry Unit 3 Exam Questions and Suggested Solutions

[Mao]

it was out of 73.

Hey, was answer to q19 MC really D? because i got that and ppl said it was wrong.

I got B but I don't doubt Mao too much aha.

Technically, B would be correct. Some of the water would vaporise and then condense as it rose up the apparatus. However, the amount of vaporised water would be very small. Practically, the difference in percentage concentration of ethanol would be so small as to be insignificant.
This should have been an explanation question.

I agree with this, I have a feeling B and D will both be accepted.
we have a resident assessor who will be/have been reading this thread, who may wish to help us argue our point

[monokekie]

does anyone know what % they had to get last year to get an A+ ? i worked out my answers think i might have got around 86%.. what will that give me?

i guess around 40?

[vamsiaus]

would that mean C would be correct because the concentration increases as ethanol rises up the column.

[avram_grant]

i put C - kinda guessed it though - i hear a lot of people didnt finish the exam... i hope the entire state found it hard because they had some left field questions - last question was a thinker... dunno why they made it so weird..

[Mao]

Upon research for MCQ 19, B is definitely correct.

Some ethanol forms an azeotrope with water (94~95% ethanol, 5~6% water). That is, this mixture does NOT separate when boiled. Hence in the vapour, there will definitely be some water.

For example, if a 50/50 mixture of ethanol and water is distilled once, the distillate will be 80% ethanol and 20% water (see ethanol data page), which is closer to the azeotropic mixture than the original. Distilling the 80/20% mixture produces a distillate that is 87% ethanol and 13% water. Further repeated distillations will produce mixtures that are progressively closer to the azeotropic ratio of 95.5/4.5%. No number of distillations, however, will ever result in a distillate that exceeds the azeotropic ratio. Likewise when distilling a mixture of ethanol and water that is richer in ethanol than the azeotrope, the distillate (contrary to intuition) will be poorer in ethanol than the original but slightly richer than the azeotrope.[2]

However, this is much beyond VCE level. And with VCE knowledge, D would seem the most obvious answer..

[note, the question itself is incorrect. The boiling point will be less than 78 degrees]

[lukeperry91]

Hmmm i answered I only for the carrier gas question, not including H2 because all my textbooks said that nitrogen gas will be used most of the time. I didn't stop to think about whether or not H2 will react with an alkane
Will that be a common error?

[mystikal]

you reckon they might give some half marks? coz that will help me alot == and if so why would they give half marks for example?

[NE2000]

Few questions:
1. Carrier gas Q: I thought that H2 doesn't react with it but as we generally always use an inert gas do you think that will be the answer? I thought about what they wanted and wondered whether they were testing our ability to remember the gases (Ne, Ar, He) that can be used for GC or...whatever...anywayz
2. Fractional distillation Q: if D is right at VCE level and B is right, then could C also be right? Rushed that one...once in a part Biology paper I think VCAA gave marks to everyone coz the question was stuffed up. Hopefully they do that again   
3. Was the stuffed up HCl put in the burette or the conical flask. Coz if in burette the titre would be larger and hence greater concentration. If in flask the titre would be smaller and hence greater concentration.
4. Went back, do you reckon they wanted Na+ and O-? Would ONa have sufficed?

That is all for now, very unhappy after getting out of the exam room but your solutions have lightened me up slightly.

[NE2000]

Oh and photosynthesis = carbon reduction (bio ftw)

[avram_grant]

hydrogen is highly flammable - so  on that fact i didnt put hydrogen as it wouldnt be smart putting a flamable gas into a hot oven.... what do you guys think?

[lukeperry91]

hydrogen is highly flammable - so  on that fact i didnt put hydrogen as it wouldnt be smart putting a flamable gas into a hot oven.... what do you guys think?

Very interesting! Any thoughts Mao?

[NE2000]

Also for the find the number of double bonds question if you stuffed up (and are still annoyed at yourself about it) part b. but use algebra to get part c. correctly (albeit in a far greater number of lines) is that perfectly OK?

[ilovevce]

hydrogen is highly flammable - so  on that fact i didnt put hydrogen as it wouldnt be smart putting a flamable gas into a hot oven.... what do you guys think?

Very interesting! Any thoughts Mao?

With a bit of extra research - H2 can be used a carrier gas and is in fact one of the most efficient for achieveing separation. However, because of its flammability and other reasons, He is often preferred. But H2 could still be used.

[methodsboy]

oh alas!
if only these answers were posted up before the exam....

[avram_grant]

hydrogen is highly flammable - so  on that fact i didnt put hydrogen as it wouldnt be smart putting a flamable gas into a hot oven.... what do you guys think?

Very interesting! Any thoughts Mao?

With a bit of extra research - H2 can be used a carrier gas and is in fact one of the most efficient for achieveing separation. However, because of its flammability and other reasons, He is often preferred. But H2 could still be used.

You think they'd expect us to know that H2 could be use? In the chem books they usually say either nitrogen or helium - and when i went to VSEC to use the GLC i didnt see H2