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Aqa chem 4/ chem 5 june 2016 thread

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Anyone know how to go about answering pH questions where it tells you that the solution has been diluted by say 990cm3?
Original post by jammypancake
Anyone know how to go about answering pH questions where it tells you that the solution has been diluted by say 990cm3?


What do you mean?
Original post by jammypancake
Anyone know how to go about answering pH questions where it tells you that the solution has been diluted by say 990cm3?



you need to divide the moles by the total volume to get the new conc of H+ and then log it to get pH (i.e. if the 990 was added to 10cm3 then the total is 1000cm3 i.e. 1dm3) but if you give an example i can explain it more clearly :smile:
Just got a few general questions about the CHEM4 content.

Do we need to know about resonance stability or the M+1 peak due to 13C? I have lost all faith in the CGP textbook.

Also, when drawing out mechanisms, do we need to include δ+/ δ− signs on the carbons/oxygens/halides?

Thanks!
Original post by jammypancake
These are really easy once you have a method to working it out:
1.) write out the equation
2.) Underneath this, write down the mol values you have for the CO and the H2 and AND the value you already have for the CO afterwards
3,) Work out the difference with the CO values (0.24-0.17=0.07)
4.) Now you know the difference for equilibrium CH3OH must be 0.07
5.) There are 2 mol of H2, multiple 0.07 by 2 (0.14), then minus the 0.014 from the original (0.38-0.14=0.24)
Sorry if this was confusing


Hi really good technique mate - however - one confusion on my end when I did this paper is got 0.07 for methanol but 0.14 for h2 as it was 1:2 ratio - so every time you get one you need to times the moles ration first then minus from the ones given ? And are they any exceptions as in Jan 10 I've seen it gain moles as h2o was a by product?




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Original post by Aethrell
Just got a few general questions about the CHEM4 content.

Do we need to know about resonance stability or the M+1 peak due to 13C? I have lost all faith in the CGP textbook.

Also, when drawing out mechanisms, do we need to include δ+/ δ− signs on the carbons/oxygens/halides?

Thanks!


The cgp complete text book is the best thing out there


Posted from TSR Mobile
Original post by Aethrell
Just got a few general questions about the CHEM4 content.

Do we need to know about resonance stability or the M+1 peak due to 13C? I have lost all faith in the CGP textbook.

Also, when drawing out mechanisms, do we need to include δ+/ δ− signs on the carbons/oxygens/halides?

Thanks!


I've never seen A Q on resonance stability - apart from knowing that the carbonyl and alkyl are most stable


Posted from TSR Mobile
Original post by ahsan_ijaz
The cgp complete text book is the best thing out there


Posted from TSR Mobile


Just not sure I trust it anymore - it got the conditions for the reduction of a nitrile wrong and the colours in CHEM5 seem to fall short of what we need to know

Original post by ahsan_ijaz
I've never seen A Q on resonance stability - apart from knowing that the carbonyl and alkyl are most stable


Posted from TSR Mobile


That's good at least. Don't think the spec mentions it at all, which is why I was curious
Original post by ssamarai
you need to divide the moles by the total volume to get the new conc of H+ and then log it to get pH (i.e. if the 990 was added to 10cm3 then the total is 1000cm3 i.e. 1dm3) but if you give an example i can explain it more clearly :smile:

I think I sort of get it, this is the official question;
Calculate the pH of the solution formed when 10.0cm3 of 0.154mol dm-3 HCl are added to 990cm3 of water
Thanks
Original post by ahsan_ijaz
Hi really good technique mate - however - one confusion on my end when I did this paper is got 0.07 for methanol but 0.14 for h2 as it was 1:2 ratio - so every time you get one you need to times the moles ration first then minus from the ones given ? And are they any exceptions as in Jan 10 I've seen it gain moles as h2o was a by product?
Yeah that's basically it, yeah I think I know the one youre talking about for Jan10. With the molecules on the RHS generally theyre gaining because theyre the products of the equilibrium and genereally the LHS lose the mol as their concentrations decrease during equilibrium
With these qs a lot of it is just trial and error sort of
I doubt we'd get one like Jan10 though again
Original post by SubwayLover1
I think there are two chiral carbons- the C with the CH3 attached the C with OH attached :smile:


thank you for your reply, I thought a chiral carbon was with four different groups attached ?? These ones only have 3 different groups
Anyone know how to answer this?
Explain why the pH of an acidic buffer solution remains almost constant despite the addition of a small amount of NaOH? (3)
Original post by jammypancake
Anyone know how to answer this?
Explain why the pH of an acidic buffer solution remains almost constant despite the addition of a small amount of NaOH? (3)


Okay so basically the added OH- ions will react with the H+ ions in buffer so equilibrium will shift to the right (to replace lost H+ ions)

Eg
HA----------------> H+ + A-
Original post by jammypancake
I think I sort of get it, this is the official question;
Calculate the pH of the solution formed when 10.0cm3 of 0.154mol dm-3 HCl are added to 990cm3 of water
Thanks



yea so you multiply 0.154 by 10/1000 to get 0.00154 moles then divide this by the total vol of 1dm3 which gives you the new conc (which in this case is the same as the moles) and then log :smile:
Original post by ssamarai
yea so you multiply 0.154 by 10/1000 to get 0.00154 moles then divide this by the total vol of 1dm3 which gives you the new conc (which in this case is the same as the moles) and then log :smile:

Thanks so much!!
Anyone have a simple way of rearranging equations? I always lose marks cos I rearrange wrong
Sneakiest topic?
Original post by Superbubbles
thank you for your reply, I thought a chiral carbon was with four different groups attached ?? These ones only have 3 different groups


There are two hydrogen's attached to each one aswell :smile:
[QUOTE="jammypancake;65698629"]
Original post by ahsan_ijaz
Hi really good technique mate - however - one confusion on my end when I did this paper is got 0.07 for methanol but 0.14 for h2 as it was 1:2 ratio - so every time you get one you need to times the moles ration first then minus from the ones given ? And are they any exceptions as in Jan 10 I've seen it gain moles as h2o was a by product?
Yeah that's basically it, yeah I think I know the one youre talking about for Jan10. With the molecules on the RHS generally theyre gaining because theyre the products of the equilibrium and genereally the LHS lose the mol as their concentrations decrease during equilibrium
With these qs a lot of it is just trial and error sort of
I doubt we'd get one like Jan10 though again


Totally agree with you - but expect the worst possible paper as the resits were generally founded easier by the majority
Original post by jammypancake
Anyone know how to answer this?
Explain why the pH of an acidic buffer solution remains almost constant despite the addition of a small amount of NaOH? (3)


Hi,

Well since your adding extra oh- ions they react with the H+ when acid dissociate
So HX= H+ + X-

When you add extra OH- ions the OH- will react with the H+ to form water thus the H+ conc is reduced at the time. However, We know the H+ conc has been reduced so the ph should drop but since we know a buffer solution is one that resists changes in Ph when small ammount of alkali/acid is added. The equilibrium will shift to the right to replace the lost H+ which will bring the pH back close to its original value

hope it helps

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