Chemistry Exam Predictions/Thoughts (1 Viewer)

[roryg03]

do you have solu for the paper?

yeah, can't post them on here though

Please refrain from posting copyrighted material, particularly those authored by private companies. Thanks.

 

[Siwel]

yeah, can't post them on here though

oh mb thought solutions for cssa were school based, im guessing now cssa make their own solutionms?

 

[XXXPUMPERXXX]

bro if phsyics and bio were more application than anything, youse reckon chem will be the same and I should spend time just doing calculations and shit?

I hope it will be like that studying module 7 and 8 makes me have suicidal thoughts

 

[izzyheslop]

pls someone clarify in NMR if the proton is in an OH group that doesn't count as an environment right? it won't be on the spectra?

 

[izzyheslop]

Le why is it not A

 

[XXXPUMPERXXX]

Why do you guys always have to study from textbooks that are 20 years old ? Anyways for the question it may be because they increase in the same ratio

 

[XXXPUMPERXXX]

pls someone clarify in NMR if the proton is in an OH group that doesn't count as an environment right? it won't be on the spectra?

It is on the spectra on this graph so it should be counted as an environment

 

[beaniebabie]

Le why is it not A

we want to be looking at the way the curves counteract the change. if you look at the curves after the reaction was subjected to a change in volume/pressure, they all counteract the change by the same amount. so it must mean that they are equal in stochiometric ratios. if that makes sense?

 

[izzyheslop]

we want to be looking at the way the curves counteract the change. if you look at the curves after the reaction was subjected to a change in volume/pressure, they all counteract the change by the same amount. so it must mean that they are equal in stochiometric ratios. if that makes sense?
View attachment 34259

thanks bestie don't mind me I am fuckin DUMB

 

[izzyheslop]

It is on the spectra on this graph so it should be counted as an environment

thanks friend I've seen it excluded in some places so wanted to make sure

 

[beaniebabie]

thanks bestie don't mind me I am fuckin DUMB

LOL definitely not dumb, i swear i've asked this question in tutoring before. so that's the only reason why i kinda knew

 

[uart]

Le why is it not A

You can also verify it's C by reading off the exact values of the concs, and then calculating the equilb constant ratios with both the initial and final values. You'll find A just doesn't work numerically. The soln given by Beanie above is a lot quicker though.

 

[TheVuk]

I hope it will be like that studying module 7 and 8 makes me have suicidal thoughts

yh hard sigmas don't fuck around with that theory bullshit

 

[CM_Tutor]

Regarding the CSSA 2021 paper, it should be noted that there were a lot of mistakes:

• In question 2, none of the equations are correct. The answer is meant to be D but there is a typo in the formula for aluminium carbonate. It should be Al₂(CO₃)₃ but has been incorrectly written as Al₂(CO₂)₃.
• Question 20 has no correct answer, which is 0.0124 mol L^-1. The answer given, C, is 0.00502 mol L^-1 is the concentration of lead ions in the solution when the precipitate first forms, but the question asked for the concentration in the aliquot.
• Not a mistake, but question 26(b) can be done with a variety of methods.
• Few people will notice, but the graph in question 27 is wrong. If you use the periods when the system is at equilibrium to estimate the equilibrium constant, you will find that it is roughly K = 0.097 to start with, decreases to K = 0.046 after the temperature change (which is fine), but then after the volume change it decreases again to K = 0.015. The movements in concentration following the volume change are in the right direction but they should be changes of roughly 0.25 mol L^-1.
• Question 33 begins by stating that isomers have formula C₄H₈O. There are quite a few possible alcohols with this MF, but I can't think of any that has a ¹H NMR spectrum like the one shown. Further, the question gives the relative areas for the five hydrogen environments as 3:3:2:1:1, which means the compound has 10 hydrogen atoms! There are only four possible alcohols with MF of C₄H₁₀O, one of which (2-butanol) matches the given NMR spectrum. I conclude that the question starts with the wrong formula and I wonder how many students were confused by this mistake and lost time.
• Question 34 asserts that "... whereas amphiprotic sodium hydrogen carbonate forms a solution with a pH of 1." This is preposterous, ridiculous, absurd, wrong, and total and complete bovine waste product. Sodium hydrogencarbonate is amphiprotic but is actually a weak base. If it truly had a pH of 1, its conjugate acid (carbonic acid, which is weak, pK_a = 6.8) would need to have a pH that was negative. Looking at the solutions, I have found that the question is meant to say that "sodium hydrogen sulfate forms a solution with a pH of 1." This is at least an acidic substance, and it is formally amphiprotic in that it has a conjugate acid and a conjugate base. However, having hydrogensulfate accept a proton and form sulfuric acid (which is strong, with a pK_a of -2.8) would require unusual circumstances. The hydrogensulfate ion has a pK_a of 2 and so would need to be present in a concentration of 1.0 mol L^-1 to have a pH of 1.

 

[CM_Tutor]

pls someone clarify in NMR if the proton is in an OH group that doesn't count as an environment right? it won't be on the spectra?

The H on OH should appear in hydrogen NMR spectra, somewhere between 1 and 5 ppm (it is difficult to predict). It generally does not cause splitting, however.

 

[CM_Tutor]

You can also verify it's C by reading off the exact values of the concs, and then calculating the equilb constant ratios with both the initial and final values. You'll find A just doesn't work numerically. The soln given by Beanie above is a lot quicker though.

You should be able to do this, but a surprising number of these types of graphs are drawn incorrectly with the final concentrations wrong. Look at the 2021 CSSA, for example.

 

[Siwel]

You should be able to do this, but a surprising number of these types of graphs are drawn incorrectly with the final concentrations wrong. Look at the 2021 CSSA, for example.

Any tips on how to know what addition or substitution reaction is occuring in reaction pathways, seem to always stuff up on this particularly with the cssa 2021 one

 

[CM_Tutor]

Addition needs an alkene. Substitution is typical for haloalkanes and alcohols undergo substitution (along with other reactions)

 

[🍑 this is a peach]

Regarding the CSSA 2021 paper, it should be noted that there were a lot of mistakes:

• In question 2, none of the equations are correct. The answer is meant to be D but there is a typo in the formula for aluminium carbonate. It should be Al₂(CO₃)₃ but has been incorrectly written as Al₂(CO₂)₃.
• Question 20 has no correct answer, which is 0.0124 mol L^-1. The answer given, C, is 0.00502 mol L^-1 is the concentration of lead ions in the solution when the precipitate first forms, but the question asked for the concentration in the aliquot.
• Not a mistake, but question 26(b) can be done with a variety of methods.
• Few people will notice, but the graph in question 27 is wrong. If you use the periods when the system is at equilibrium to estimate the equilibrium constant, you will find that it is roughly K = 0.097 to start with, decreases to K = 0.046 after the temperature change (which is fine), but then after the volume change it decreases again to K = 0.015. The movements in concentration following the volume change are in the right direction but they should be changes of roughly 0.25 mol L^-1.
• Question 33 begins by stating that isomers have formula C₄H₈O. There are quite a few possible alcohols with this MF, but I can't think of any that has a ¹H NMR spectrum like the one shown. Further, the question gives the relative areas for the five hydrogen environments as 3:3:2:1:1, which means the compound has 10 hydrogen atoms! There are only four possible alcohols with MF of C₄H₁₀O, one of which (2-butanol) matches the given NMR spectrum. I conclude that the question starts with the wrong formula and I wonder how many students were confused by this mistake and lost time.
• Question 34 asserts that "... whereas amphiprotic sodium hydrogen carbonate forms a solution with a pH of 1." This is preposterous, ridiculous, absurd, wrong, and total and complete bovine waste product. Sodium hydrogencarbonate is amphiprotic but is actually a weak base. If it truly had a pH of 1, its conjugate acid (carbonic acid, which is weak, pK_a = 6.8) would need to have a pH that was negative. Looking at the solutions, I have found that the question is meant to say that "sodium hydrogen sulfate forms a solution with a pH of 1." This is at least an acidic substance, and it is formally amphiprotic in that it has a conjugate acid and a conjugate base. However, having hydrogensulfate accept a proton and form sulfuric acid (which is strong, with a pK_a of -2.8) would require unusual circumstances. The hydrogensulfate ion has a pK_a of 2 and so would need to be present in a concentration of 1.0 mol L^-1 to have a pH of 1.

there were sooo many mistakes ... do they even bother proof reading it?
and there were mistakes in other cssa papers too, in software there was a question not even relevant to the syllabus

 

[CM_Tutor]

there were sooo many mistakes ... do they even bother proof reading it?
and there were mistakes in other cssa papers too, in software there was a question not even relevant to the syllabus

This one is unusually bad for CSSA standards. Checking thoroughly is not that easy, either - many people wouldn't notice a detail like the mistake in q2 or the flaw in the equilibrium graph.