VCAA 2012 Chemistry Unit 3 Exam Discussion

[lokdog]

for 13 I got B, I used c1v1=c2v2, what did i do wrong

[HERculina]

Is solubility sam as concentration but in g/L? I did what i got for he previous q. Multiplied by molar mass ane divided it by thr volume (L) . Can't rmb the actual values
Ya i got C for 12 - just assumed double peak area but same retention time
The spec q.with the three tablets - wen it asks find the avergage mass in mg, would you average the absorbance values and then locate concenttation for that?

[aznxD]

for 13 I got B, I used c1v1=c2v2, what did i do wrong

V2 would be 75mL not 60mL

[sahil26]

for 13 I got B, I used c1v1=c2v2, what did i do wrong

Maybe you didn't realise that the volumes were in ml... or (more likely) ... u didn't substitute ur numbers at the right place

[jadams]

In the tablet one, did it ask for average? or mass in each tablet..... (of iron)

[thushan]

Just did the paper, here are my thoughts.

Overall, relatively decent. MC was very easy, SA not too bad.

SAQ1 - Pedestrian.
SAQ2 - Good original question, 2c will need some good explanation.
SAQ3 - Decent. Little nervous about functional group qn. I put down amide then peptide. 3c will also need sound explanation.
SAQ4 - Excellent question. 4c I'm impressed with, 4d very impressed with. Personally i think either GC or IR should be fine for 4.
SAQ5 - Disappointed. NMR question way too easy.
SAQ6 - Decent qn requiring explanation of principles.
SAQ7 - Pedestrian qn, solubility data unnecessary. For those worried about whether solubilities should be a factor in calculations, they aren't in this case because you don't know the volume of KI solution nor do you know the volumes of water that you washed with.
SAQ8 - Good back titration question, 8b is quite tricky.

Will write solutions on these a little later today, I have to keep writing my book!

A+ cut prediction 67/77.

[sahil26]

In the tablet one, did it ask for average? or mass in each tablet..... (of iron)

I thought the question said "tablets" so i did all three :/

[Tonychet2]

my suggested answers! suggested answers.. written by me lol

[pi]

Hey Thushan, I'll race you for solutions?

WOOOOO! GO ILLUMINATI!

[ezye21]

for the pure sample i suggested fractional distillation, and i gave the mass of each tablet in mg, overall  though the exam wasnt hard but i still believe people made mistakes that they didn't see or dont think they have made. i ran out of time nothing on the exam that you couldn't do just time wise i ran out -.- fml cant wait for suggested solutions any one reckon the A+ cutt off with be around 65/75 ?

[acinod]

1.a.i. Glucose
1.a.ii. C6H12O6(aq)---(enzymes)--->2C2H6O(aq)+2CO2(g)
1.a.iii. C2H4(g)+H2O(g)-----H3PO4/300 degrees celcius----->C2H6O(aq)
1.b.i. 3 and 1
1.b.ii. Stearic Acid
1.b.iii. CH3(CH2)16COOH(aq)+26O2(g)->18CO2(g)+18H2O(l)
2.a. Tyrosine
2.b. It's the lower middle dot.
2.bc. Because you only used one solvent in chromatogram I which gives 3 dots but in chromatogram II you used two solvents which give 4 dots.
3.a. Can't really draw the structure on a computer but: H2N-(CH2)6-NH2
3.b. carboxy, amino
3.c. In nylon, the two individual monomers have the same functional groups on both ends. Hence the orientation of the linkage will alternate between CONH and NHCO. However in protein, the two individual monomers have different functional groups on both ends and so the orientation of the linkage will always be CONH.
3.d. Like I said can't really draw but: -H2C-C(CH3)(COOCH3)-CH2-C(CH3)(COOCH3)-
4.a. Propan-1-ol, propanoic acid
4.b. CH3CH2CH2OH(aq)+CH3CH2COOH(aq)----H2SO4--->CH3CH2COOCH2CH2CH3(aq)+H2O(l)
4.c. You start with propan-1-ol then you make propanoic acid by oxidising the propan-1-ol using an oxidant such as potassium permanganate. Then you react the propan-1-ol and propanoic acid in the presence of sulfuric acid to form propyl propanoate. Then you seperate the propyl propanoate from everything else using fractional distillation.
4.d. Add potassium dichromate. If it changes colour, then it's not pure because there's propan-1-ol. Add sodium carbonate. If gas produced (bubbles), then it's not pure because there's propanoic acid. Or you can use HPLC and see if it only produces a peak with a retention time that propyl propanoate is at (determined by known standards).
5.a.i. 2
5.a.ii. 3
5.a.iii. 6
5.b. OH (alkanol)
5.c. Draw full structure of Propan-2-ol (CH3CH(OH)CH3)
6.a.i. Plot points
6.a.ii. 35mg
6.b.i. UV-Visible
6.b.ii. Absorbed energy causes electrons to move from lower energy levels to higher energy levels.
7.a.i. Pb(CH3COO)2(aq)+2KI(aq)->PbI2(s)+2KCH3COO(aq)
7.a.ii. To get a constant mass.
7.a.iii. 0.0939g
7.a.iv. 0.331g
7.b. Nitrates are soluble.
8.a.i. OH-(aq)+H+(aq)=H2O(l)
8.a.ii. 4.00*10^-3 mol
8.a.ii. 2.70*10^-2 mol
8.a.iii. 289g/L
8.a.b. It will decrease. The acid will be diluted, hence more acid will be required. As a result, your excess would increase and so reacting would decrease. Thus the amount of ammonium chloride would decrease and hence the calculated solubility of ammonium chloride will decrease.

[joseph95]

There are 2 carbon environments not 3?
And your oxidation of stearic acid is strange
6.a.ii. is 35/36mg dewd. Why you got so many zeroes boi

[Shenz0r]

It was a pretty easy exam, but I'm paranoid that since the A+ mark is so going to be so high that it's going to be extremely difficult to get into the high 40s.

[acinod]

There are 2 carbon environments not 3?
And your oxidation of stearic acid is strange
6.a.ii. is 35/36mg dewd. Why you got so many zeroes boi

Oops my bad there are only 2 carbon environments and yea the oxidation equation i forgot to count some hydrogens so hopefully it's fixed up.
And oops my bad it is 35mg (accidently divided by 0.1 instead of multiplied...would be weird having 3.5g of iron in a tablet LOL must be that super pill from dragonballZ)

[ezye21]

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