General thoughts: HSC chemistry 2015 (3 Viewers)

[Bozzin8R]

Confirm, I had something like that

splendid haha, went from people telling me I was wrong because of something to do with ACETIC acid to now hopefully being right - bumps my short answers up by 2 marks which is always nice

 

[Librah]

Why was that worth 4 marks?

3(0.1)(0.025)/0.0415=0.181 M

 

[Bozzin8R]

I got 0.181 (3sf.) lol
Was the question something like 25mL citric at 0.100mol/L reacts with sodium hydroxide and avg sodium hydroxide was 41.5mL?

The only reason I'm thinking this may be wrong is by looking at it logically - if 25mL of 0.1mol citric (weak) acid needed 41.5mL of NaOH (strong), it would make sense that the NaOH would be more dilute than this acid? Correct me if Im wrong here

 

[kawaiipotato]

The only reason I'm thinking this may be wrong is by looking at it logically - if 25mL of 0.1mol citric (weak) acid needed 41.5mL of NaOH (strong), it would make sense that the NaOH would be more dilute than this acid? Correct me if Im wrong here

That sounds logical. But they gave amount of citric needed to neutralise it anyway so wouldn't this already mean it takes into account the weakness of it?

C6H8O7 + 3NaOH ---> Na3C6H5O7 + 3H2O

nC6H807 = 0.1*0.025 = 0.0025
nNaOH = 0.0025*3 = 0.0075
[NaOH] = 0.0075/0.0415 = 0.108mol/L
I have a feeling I just lost out on 4 marks now

 

[Librah]

The only reason I'm thinking this may be wrong is by looking at it logically - if 25mL of 0.1mol citric (weak) acid needed 41.5mL of NaOH (strong), it would make sense that the NaOH would be more dilute than this acid? Correct me if Im wrong here

wat

 

[Bozzin8R]

wat

Dw guys you are right I think I've just got this all mixed up

 

[Librah]

Citric acid has 3 Carboxylic acid groups iirc? Donates 3 protons.

 

[Ekman]

splendid haha, went from people telling me I was wrong because of something to do with ACETIC acid to now hopefully being right - bumps my short answers up by 2 marks which is always nice

Sorry, I just looked through the question again and I got the same answer as librah (I remember the multiplying the moles by 3 to get 0.0075, and dividing by 0.0415 to get 0.181).

I must of remembered 0.006 from some other question I did last night.

Edit: I remembered it from the 2014 hsc paper I did last night, where the answer to question 30 a was 0.005

 

[malcolm21]

for that citric acid one i forgot it was triprotic thought it was mono for some reason but did the working out and got the answer off by a 3 factor, what do you think ill get for it, its bullshit that was a 4 marker, 2 or 3 marker at most man

 

[CookieMan999]

I picked B for the polymer in Q11 because as in glucose, two hydroxyl groups (OH) condense to expel a H2O and leave only ONE oxygen atom to form the link. None of the other options display this.

D looked correct at first but having an oxygen on both sides would mean that they had condensed to form H2 (similarly for C, 2 oxygens are in the middle for the link instead of one from condesation). A proper answer should have had D but with one less O on one side.

 

[HecticLad]

View attachment 32594

I picked B for the polymer in Q11 because as in glucose, two hydroxyl groups (OH) condense to expel a H2O and leave only ONE oxygen atom to form the link. None of the other options display this.

D looked correct at first but having an oxygen on both sides would mean that they had condensed to form H2 (similarly for C, 2 oxygens are in the middle for the link instead of one from condesation). A proper answer should have had D but with one less O on one side.

Im probabaly thinking way too much into it, but maybe it wasnt necessarily H2O that was condensed out, as condensation polymerization can refer to the release of any small molecule, not just H2O, maybe it could have been H2 or methanol instead although, but otherwise, all the options on there are technically wrong, could just send some complaints in about it tbh

 

[BillKyriakos]

What do you guys think an 85 would align to?

 

[nerdz1]

Im probabaly thinking way too much into it, but maybe it wasnt necessarily H2O that was condensed out, as condensation polymerization can refer to the release of any small molecule, not just H2O, maybe it could have been H2 or methanol instead although, but otherwise, all the options on there are technically wrong, could just send some complaints in about it tbh

Yeah I think you're thinking way into it lol. BOS generally never extends into condensation with H2 or methanol product.

 

[nerdz1]

What do you guys think an 85 would align to?

This won't be helpful but tbh nobody can say definitively. Even though the general consensus seems to be that it was an "easy" paper, we still need to see how the state goes holistically. Also mark alignment can change dramatically from year to year, from memory, 85 went to 95 in one year whilst 90 went to 94 in another year. So yeah, it's a little bit hard to predict.

 

[BillKyriakos]

This won't be helpful but tbh nobody can say definitively. Even though the general consensus seems to be that it was an "easy" paper, we still need to see how the state goes holistically. Also mark alignment can change dramatically from year to year, from memory, 85 went to 95 in one year whilst 90 went to 94 in another year. So yeah, it's a little bit hard to predict.

That's fair. Thanks for the response

 

[iforgotmyname]

I got 10.6?

For the industrial question, Moles were

0.5 + 1 -> 2.5. Where the one with 0.5 mole has 2 in front and rest has 1 in front

It said the one with 0.5 moles had change (increase) in concentration of 0.36 moles.

This will mean that the one with 1 mole will increase by 0.36x2 (due to the ratio being 2:1 between 0.5 molar and 1molar substance on lhs) This will also mean that (00.36x2) 2.5 moles product will be consumed to make the lhs reaction.

You divide everything by 2 thus yielding you

0.43 + 0.86 -> 0.89 thus equilibrum constant will be around 5.59

Idk is this right or did I fk up the molar ratio

 

[becauseiambored]

Clean full pdf version of 2015 Chemistry HSC Paper.

https://www.dropbox.com/s/srhwrj25tmu7jbj/hsc_chem_2015.pdf?dl=0

 

[Vizsla]

If i left my answer as 0.1807 (stressing out bout this) will i lose a mark as i think its one too many sf

 

[BlueGas]

If i left my answer as 0.1807 (stressing out bout this) will i lose a mark as i think its one too many sf

What question was this again? I remember I got the same answer and I left the answer as 0.18

 

[Vizsla]

What question was this again? I remember I got the same answer and I left the answer as 0.18

NaOH conc