2009 Chemistry Unit 3 Exam Questions and Suggested Solutions

[Mao]

ey Mao, you sure about Qstn 14 of multiple choice?? id belive it to be C, the diagram is a bit tricky...

tertiary structure is the overall shape of the protein due to interactions of Z groups. In this case, II is only interaction between the peptide linkages [which forms alpha helix or beta pleating], the over jumbled up structure in IV is the tertiary structure.

parts of IV are alpha helices, which are secondary structures, but the overall shape of IV shows the tertiary structure.

[Mao]

Hmm I haven't read through everything so maybe this has already been asked, but you didn't include Step 1 in the short answer Q7.
This is CO2 going to C6H12O6
The C goes from oxidation state +4 to 0, and this is therefore a reduction. unless there is something I am missing about "overall" reduction

Check Errata in first post

[ilovevce]

Further to what Mao said, H2 is in fact used in flame ionisation detectors in the gas chromatograph anyway. An explosion is avoided by controlling the flow of oxygen into the flame ionisation detector.

Even if there were small amounts of oxygen or reactive impurities in the sample, combustion would not occur. If the volume concentration of H2 is greater than about 75%, uncontrolled combustion does not occur as there are simply not enough collisions between H2 and other molecules.

[littlecherry25]

CAN SOMEONE EXPLAIN QUESTION 19 TO ME IN MC？？

[ilovevce]

As for short answer Question 3 c (titration), I just realised I didn't read the question properly. Indeed the question did not specify whether NaOH was in the burette or conical flask. Check first post for additional explanation

Wouldn't it be convention that the HCl solution (with NaOH added) is in the burette? Usually the solution of unknown concentration is in the conical flask, and the solution of known concentration is in the burette.

[eccentric]

sigh ..... what you guys reckon the A+ cut off is ? i reckon its gonna be 63/73

Wasn't last year's cut off about 87% ?

[homghomg1]

Hmm I haven't read through everything so maybe this has already been asked, but you didn't include Step 1 in the short answer Q7.
This is CO2 going to C6H12O6
The C goes from oxidation state +4 to 0, and this is therefore a reduction. unless there is something I am missing about "overall" reduction

Check Errata in first post

Thanks, didn't see it. so we've concluded that Q19 is B and not D? If it's D I think I'm looking at 70/73

[eccentric]

2008 mid years: A score of approximately 87 per cent was needed to achieve an A+ and approximately 80 per cent was needed to receive an A. The mean score for this examination was 58.3 per cent, compared to 58.9 per cent in 2007. The mean score corresponded to a grade of a high C+.

[Niksko]

regarding MCQ 4, wouldn't the two OH groups count as an identical hydrogen environment because at this level we're taught to only take into account adjacent atoms when looking at whether a hydrogen environment is different, so therefore you would only get 3 sets of peaks?

[littlecherry25]

regarding MCQ 4, wouldn't the two OH groups count as an identical hydrogen environment because at this level we're taught to only take into account adjacent atoms when looking at whether a hydrogen environment is different, so therefore you would only get 3 sets of peaks?

there is only one OH group
the other group is a COOH group

[homghomg1]

Short answer Q2 b)
Hmm I only wrote about how salicylic acid and aspirin have different structures and will therefore have different retention times. I didn't put it together with the UV light information. I thought that was just to trick you, or maybe relevant to the next question and they put it there before for no reason. Count myself as losing 1 mark there. What Mao wrote was pretty smart, I doubt many people will be able to put the information together and come up with something similar. Hard question.

[simpak]

Short answer Q2 b)
Hmm I only wrote about how salicylic acid and aspirin have different structures and will therefore have different retention times. I didn't put it together with the UV light information. I thought that was just to trick you, or maybe relevant to the next question and they put it there before for no reason. Count myself as losing 1 mark there. What Mao wrote was pretty smart, I doubt many people will be able to put the information together and come up with something similar. Hard question.

I wrote about standards and calibration curves for concentration of salicylic in the product

[TrueTears]

Hmm... would it be ok for q 2d) if you didn't put the bond between and ? ie,

[spizaa #5]

58/73 .. rope out X(
                         

[simpak]

I put the Na on the other branch?
I was confused.
I replaced a hydrogen from the CH3 with the Na and I have NO IDEA WHY.
This is what happens when you have to rush.  Stupid length of exam booklet, you kill me.