AQA CHEM4 and CHEM5 revision thread 2016

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Could anyone explain this buffer question?

It has to be a salt of base as this is what makes up the [CH3CH2NH3+] in the Ka equation if that makes sense

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Reply 101

RedDevil1997

Can anyone explain how to do the ph questions when the acid is diluted, for example 'Calculate pH when 10cm3 of 0.154M HCL is added to 990cm3 of water' Thanks in advance! 😀

Reply 102

B_9710

Original post by RedDevil1997

Can anyone explain how to do the ph questions when the acid is diluted, for example 'Calculate pH when 10cm3 of 0.154M HCL is added to 990cm3 of water' Thanks in advance! 😀

Calculated miles of H+ from the acid. Then divide by the new volume, which in this case would be 1dm^3, this gives you [H+]. Then just calculate pH.

Reply 103

AccountAtL

Does anyone know why D2O is not an accepted solvent in H NMR? I just did June 2013 past paper and it was wrong. CCl4 and CDCl3 were accepted. Is it to do with the solubility of the sample (dichlorodimethylbutane)? Thanks

Reply 104

ahsan_ijaz

any one have any tips with proton nmr - i struggle with it when you have to predict structures - ive practiced a lot but its something im trying to do since a long time but maybe their may be some tricks that help with it?

Reply 105

AccountAtL

Original post by ahsan_ijaz

It's a difficult one as I guess some people find it easier to just 'see' the answer. Try to go through it systematically. Draw out the structure from the data table associated with each ppm (then use the integration (tells you how many / the proportion of Hs in that environment) and splitting (tells you how many Hs are on the adjacent C)). Once you have done that try and piece them together like a jigsaw, to match the molecular formula given. Also, if it is taking you too long, get the down marks you can and move on. If you have time you can come back to it at the end, but if you know it is a question you struggle with don't waste too much time for the sake of a couple of marks you could more easily get elsewhere. Good luck.

(edited 7 years ago)

Reply 106

ahsan_ijaz

Original post by AccountAtL

It's a difficult one as I guess some people find it easier to just 'see' the answer. Try to go through it systematically. Draw out the structure from the data table associated with each ppm (then use the integration (tells you how many / the proportion of Hs in that environment) and splitting (tells you how many Hs are on the adjacent C)). Once you have done that try and piece them together like a jigsaw, to match the molecular formula given. Also, if it is taking you too long, get the done marks you can and move on. If you have time you can come back to it at the end, but if you know it is a question you struggle with don't waste too much time for the sake of a couple of marks you could more easily get elsewhere. Good luck.

Hi,

i find chem 4 easy due to its nature of all memory and exam technique - they cant really trick you much apart from us not reading Qs properly or making stupid mistakes. I due generally spend a lot more time on nmr when i need to predict structures - im fine with picking out enviroments and stuff

Reply 107

yung7up

Can anyone explain qualitatively how buffers work? I roughly know what happens but the wording with this question catches me every time

Plus when do we minus the moles in ph questions to find the XS? Is that just for buffers?

Thanks

Reply 108

ahsan_ijaz

But then again chem 5 is much more tricky as they can mess you up with stupid errors we can make in exam conditions

Reply 109

Ninjakam119

Original post by ahsan_ijaz

Not really many tricks, probably the most tedious part of chemistry, but one thing which may seem obvious but does help is when you identify the CH3 group(s), put them on the ends (as they obviously can't be in the middle) and try to work inwards.

Reply 110

twallien

Original post by AccountAtL

Yeah, you need a polar solvent in order to dissolve the sample. If you imagine trying to put oil (long chain hydrocarbons) in water, it wont mix. D2O is basically water (Deuterium being an isotope of Hydrogen).

Reply 111

twallien

Original post by yung7up

Buffer solution is an equilibrium. By adding H+ or OH- ions, they react and the equilibrium shifts to the side to reduce the change, meaning if only small quantities are used, the equilibrium will shift enough that the pH change in very small.

At the start of the question write out the equation. Calculate the number of moles of H+ and OH-, which ever has the most moles is in excess. Also regardless of which is in excess, you need these values for your calculation. Watch out for diprotic acids though!

Reply 112

yung7up

Original post by twallien

But doesnt the explanation differ depending on whats being added e.g when you OH- ions they react with H+ form water so eqbm opposes this blah blah but what happens when you add the acid? It has something to do with the ratio of the salt to the acid or something like that im not too sure

OHH ok so regardless of whether its in excess i still need it for Ka or Kw or whatever and then im assuming you caluclate the conc (if need to) using the new volume?, plus is this just for buffer solutions or neutralisation reactions in general?

Thank you very much btw

Reply 113

AccountAtL

Original post by twallien

Thanks! But isn't CCl4 non polar too (i.e. there are polar bonds but it is symmetrical so there the molecule itself isn't polar)?

Reply 114

twallien

Original post by _bob_

Could anyone explain this buffer question?

A buffer is an equilibrium, i'll use HX as an example:
HX <--> (H+) + (X-)

If you add an acid, it reacts with (X-) and equilibrium shifts to the right to replace the lost ions. Same with a base, but instead it reacts with H+ ion. Now in order for the buffer to work with either case, you need a case where [H+] =/= [X-]. You get this by adding the salt of the acid, (NaX).

This is the same with the basic buffer, you need more of the products of the equilibrium, so you need the salt. With the amino groups the salt is a Quaternary ammonium salt.

Reply 115

twallien

Original post by yung7up

Yes it differs, but it is literally down to Le Chettaliers principle. I'll use the acid HX as an example:

HX <--> (H+) + (X-)

If you add an acid, it will cause the equilibrium to shift to the left (to reduce the concentration of H+ ions) this will mean the H+ ions will react with X- and produce more HX and reducing [H+]. Now you add a base; H+ will react with OH- and produce water, this time the equilibrium will shift to the right to reduce the loss of H+ but dissociating HX thereby restoring [H+] (mostly, there will be a small change in [H+] but less so than if it wasn't a buffer)

Reply 116

twallien

Original post by AccountAtL

Thanks! But isn't CCl4 non polar too (i.e. there are polar bonds but it is symmetrical so there the molecule itself isn't polar)?

Sorry i've got that completely wrong, i'm at a loss with this one, sorry!

Reply 117

ManOfJustTin

Original post by AccountAtL

Thanks! But isn't CCl4 non polar too (i.e. there are polar bonds but it is symmetrical so there the molecule itself isn't polar)?

I think they use D20 to test for impurities rather than as a solvent. If you used D20, any OH peak would disappear as it is replaced by the deuterium so it can't be used.