HSC Chemistry Question Thread

[Fahim486]

Hey so I just wanted to know why the answer is D and not A. Thanks!

[Natasha.97]

Hey so I just wanted to know why the answer is D and not A. Thanks!

Hi!

There is a "lowest sum of locants" rule when naming these compounds. The sum of locants on D is 7 (2+2+1+1+1), as opposed to 8 (1+1+2+2+2) for A.

Hope this helps

[mary123987]

Hey guys, just wondering is there like any formula or something to calculate the number of isomers?
like for example a question is asking :how many isomers are there for C3H6Cl2
these are the combinations i found (1,1), (1,2) ,(1,3),(2,2) ,(2,3)
the options are as listed below:
a) 3
b) 4
c) 5
d) 6
the answer is b!

[arunasva]

Hey guys, just wondering is there like any formula or something to calculate the number of isomers?
like for example a question is asking :how many isomers are there for C3H6Cl2
these are the combinations i found (1,1), (1,2) ,(1,3),(2,2) ,(2,3)
the options are as listed below:
a) 3
b) 4
c) 5
d) 6
the answer is b!

if you can derive one :')

[Shadowxo]

Hey guys, just wondering is there like any formula or something to calculate the number of isomers?
like for example a question is asking :how many isomers are there for C3H6Cl2
these are the combinations i found (1,1), (1,2) ,(1,3),(2,2) ,(2,3)
the options are as listed below:
a) 3
b) 4
c) 5
d) 6
the answer is b!

I don't know of any formulas, for questions like this it's really guess-and-check
You have to remember that (1,2) is the same as (2,3) as you can rotate the first to get the second.

I'd approach this question by approaching it like you have, finding where the two Cl atoms could be. They could both be on the first carbon, or both be on the second/middle, but both being on the third is the same as both being on the first. So two options if they're both on the same carbon atom. If they're not, they could be one on either side, or one on the middle and one on the side, so two options (as having one on the middle and one on the other side is the same as the second option). This would give a total of 4.

You just need to figure out which options aren't redundant, if there's a formula I don't know of it

[angelahchan]

Hey guys, just wondering is there like any formula or something to calculate the number of isomers?
like for example a question is asking :how many isomers are there for C3H6Cl2
these are the combinations i found (1,1), (1,2) ,(1,3),(2,2) ,(2,3)
the options are as listed below:
a) 3
b) 4
c) 5
d) 6
the answer is b!

Was this in an HSC paper? Because I don't you need to know how many isomers random organic compounds have (I think the syllabus point is more about naming haloalkanes) . I did a google, and no there isn't a formula for organic compounds in general, but there is one for alkanes if you're interested:  (it looks incredibly complicated)
http://pubs.acs.org/doi/abs/10.1021/ci00027a019?journalCode=jcics1&

[mary123987]

I don't know of any formulas, for questions like this it's really guess-and-check
You have to remember that (1,2) is the same as (2,3) as you can rotate the first to get the second.

I'd approach this question by approaching it like you have, finding where the two Cl atoms could be. They could both be on the first carbon, or both be on the second/middle, but both being on the third is the same as both being on the first. So two options if they're both on the same carbon atom. If they're not, they could be one on either side, or one on the middle and one on the side, so two options (as having one on the middle and one on the other side is the same as the second option). This would give a total of 4.

You just need to figure out which options aren't redundant, if there's a formula I don't know of it

Yes !!! that's what i forgot thankyou soooo much i completely forgot about how 2,3 is the same as 1,2 (when you look from the oppsote side) wow good spot thanks a lot

Was this in an HSC paper? Because I don't you need to know how many isomers random organic compounds have (I think the syllabus point is more about naming haloalkanes) . I did a google, and no there isn't a formula for organic compounds in general, but there is one for alkanes if you're interested:  (it looks incredibly complicated)
http://pubs.acs.org/doi/abs/10.1021/ci00027a019?journalCode=jcics1&

Hey yes this is actually from the 2006 paper q5
 Haha that formula looks intense like 6 unit maths!!http://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2006exams/pdf_doc/chemistry_06.pdf

[winstondarmawan]

Yes !!! that's what i forgot thankyou soooo much i completely forgot about how 2,3 is the same as 1,2 (when you look from the oppsote side) wow good spot thanks a lotHey yes this is actually from the 2006 paper q5
 Haha that formula looks intense like 6 unit maths!!http://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2006exams/pdf_doc/chemistry_06.pdf

The best bet for the amount of isomers I think is to draw them out. It's long and tedious but ensure you don't skip one or have an extra one. Do this at the end of the exam.

Anyways, a question.
When ammonia is added to water, does the pH increase or decrease?
Thanks in advance.

[Fahim486]

Hey so for question 4 the answer is C but I got B and don't understand why, for question 10 why is the answer A and not D and finally for question 20 I got B but the answer is D and can't figure out why and I'm thinking maybe I jumbled my calculations or something. Thanks!

[mary123987]

Hey so for question 4 the answer is C but I got B and don't understand why, for question 10 why is the answer A and not D and finally for question 20 I got B but the answer is D and can't figure out why and I'm thinking maybe I jumbled my calculations or something. Thanks!

Hey so im currently answering q 20 i got D
so the molar heat of combustion is equal to q/n
q=mcΔT
n=m/M
mplar heat of combustion =mc(ΔT)/(m/M)
firstly q=m(4.18)(45-20)
n=1/116.204
now heptanol has 7 carbons and so
4638=m((4.18)(25)) /(1/116.204)
therefore m(4.18(25) =4638(1/116.204)
therefore m=0.3819384455kg
therefore d
let me know if you are confused

[MisterNeo]

Hey so for question 4 the answer is C but I got B and don't understand why, for question 10 why is the answer A and not D and finally for question 20 I got B but the answer is D and can't figure out why and I'm thinking maybe I jumbled my calculations or something. Thanks!

For question 4, oxidation is when something loses electrons to become more positive. Since the Fe²⁺ becomes a more positive  charge to Fe³⁺, it becomes oxidised by losing the electrons to be more positive.
For question 10, answer D is unbalanced.

[mary123987]

The best bet for the amount of isomers I think is to draw them out. It's long and tedious but ensure you don't skip one or have an extra one. Do this at the end of the exam.

Anyways, a question.
When ammonia is added to water, does the pH increase or decrease?
Thanks in advance.

HEY from my understanding ammonia is basic thus by adding ammonia to water the pH should increase (remember acids decrease pH and bases increase pH)
this is apparent by its reaction with water NH3 + H2O -->  NH4+ + OH-
its clear OH- is a product thus making it basic
let me know if your confused

[Fahim486]

Hey so im currently answering q 20 i got D
so the molar heat of combustion is equal to q/n
q=mcΔT
n=m/M
mplar heat of combustion =mc(ΔT)/(m/M)
firstly q=m(4.18)(45-20)
n=1/116.204
now heptanol has 7 carbons and so
4638=m((4.18)(25)) /(1/116.204)
therefore m(4.18(25) =4638(1/116.204)
therefore m=0.3819384455kg
therefore d
let me know if you are confused

It's sort of starting to kind of make sense but I originally thought you sub in the temperature and heat of combustion into the q=mcAt formula and just rearrange it to make m the subject which is probably why I ended up getting B as the answer but one thing I wanted to ask how did you know you had to use the molar heat of combustion thing. Thanks!

[mary123987]

It's sort of starting to kind of make sense but I originally thought you sub in the temperature and heat of combustion into the q=mcAt formula and just rearrange it to make m the subject which is probably why I ended up getting B as the answer but one thing I wanted to ask how did you know you had to use the molar heat of combustion thing. Thanks!

Hey so like you i always thought that you just use q=mcΔT but i found out that q refers to the amount of heat realeased , when asked for the molar heat of combustion or heat of combustion(really the same thing ) they are asking for the amount of energy(in the form of heat) given off by 1 mol of the substance now how i knew this is by looking at the table as the units are kJ/mol.
Since they have already given you the value of ΔHc/mol it is just a matter of manipulating the formula to get what you want in this case m (remember they dont give you something unless you need it ).
If you are still confused about anything please let me know ,hope it makes sense

[Fahim486]

Hey so like you i always thought that you just use q=mcΔT but i found out that q refers to the amount of heat realeased , when asked for the molar heat of combustion or heat of combustion(really the same thing ) they are asking for the amount of energy(in the form of heat) given off by 1 mol of the substance now how i knew this is by looking at the table as the units are kJ/mol.
Since they have already given you the value of ΔHc/mol it is just a matter of manipulating the formula to get what you want in this case m (remember they dont give you something unless you need it ).
If you are still confused about anything please let me know ,hope it makes sense

Ahh okay that makes a lot more sense now. Thanks!