x Turn on thread page Beta
 You are Here: Home

# OCR A2 CHEMISTRY F324 and F325- 14th and 22nd June 2016- OFFICIAL THREAD watch

1. (Original post by itsConnor_)
lol I literally just finished the paper and marked it today as he posted it too! Got 79 :s 87 for A* :O
I'm terrified that I'll make a mistake on questions like where they added Mg. Or get a last redox titration one wrong because I'll be rushing it because of time
2. (Original post by tcameron)
attached question 6:

with q7:
moles KMnO4- = 0.02x0.0234 = 4.69x-4 moles
moles H202 is therefore 4.69x-4 x 5/2 due to the 5:2 ratio in the redox equation so = 1.1725x-3 moles
need to x10 to get this in 250cm3 from 25cm3 so = 0.011725moles
to find mass (because the answer is needed in gdm-3) need to times the moles by the Mr of H202 which is 0.011725x34 = 0.39865g
concentration will then be 0.39865/0.025 to get is as 15.9 gdm-3
moles of oxygen will be the diluted moels of H202 so is 1.1725x10-3 x24 = 0.028dm3 for the volume of oxygen

Thank you so much. I was wondering though- so can you use gdm-3 as moles? Cuz in both questions that's what it seems has happened???
3. If it asked to draw a diagram of how you would measure the standard electrode potential of this Cl2 half cell would this be right??

EDIT: and would the electromotive force be just 1.36V?
EDIT2: and would overall equation be Cl2 + H2 --> 2H+ +2Cl-?
EDIT3: and would pH of solution in the hydrogen one DECREASE as the reaction progressed as conc. of H+ decreased sicne its electropotential is more negative than chlorine?
Posted from TSR Mobile
Attached Images

4. (Original post by Dentistry101)
Thank you so much. I was wondering though- so can you use gdm-3 as moles? Cuz in both questions that's what it seems has happened???
no they're different
5. (Original post by itsConnor_)
If it asked to draw a diagram of how you would measure the standard electrode potential of this Cl2 half cell would this be right??

EDIT: and would the electromotive force be just 1.36V?
EDIT2: and would overall equation be Cl2 + H2 --> 2H+ +2Cl-?
EDIT3: and would pH of solution in the hydrogen one DECREASE as the reaction progressed as conc. of H+ decreased sicne its electropotential is more negative than chlorine?
Posted from TSR Mobile
All correct apart from the pH value of the hydrogen would decrease due to an increase in H+ concentration. The higher the concentration of H+ ions = a lower pH.

The standard hydrogen half cell is 2H+ + 2e- <=> H2 and as this is less positive than the chlorine half cell it is reduced. Therefore H+ ion concentration would increase.
6. (Original post by VMD100)
All correct apart from the pH value of the hydrogen would decrease due to an increase in H+ concentration. The higher the concentration of H+ ions = a lower pH.
how do we know H+ would decrease exactly though? I know I said it just don't rly understand

Also if we EVER have a gas in an electropotential, we draw it as a hydrogen half cell BUT with the gas going in instead of the hydrogen? Never seen an OCR A question like this before :s
7. (Original post by itsConnor_)
how do we know H+ would decrease exactly though? I know I said it just don't rly understand

Also if we EVER have a gas in an electropotential, we draw it as a hydrogen half cell BUT with the gas going in instead of the hydrogen? Never seen an OCR A question like this before :s
I am not sure what you mean by the first part? The H+ concentration is increasing not decreasing?
And while I have never seen an OCR question with another gas other than hydrogen yes if it is gaseous you would need the 1 atmosphere/100kPa, which is really 101kPa and for it to be bubbled into the aqueous solution of its ions.
8. (Original post by VMD100)
I am not sure what you mean by the first part? The H+ concentration is increasing not decreasing?
And while I have never seen an OCR question with another gas other than hydrogen yes if it is gaseous you would need the 1 atmosphere/100kPa, which is really 101kPa and for it to be bubbled into the aqueous solution of its ions.
why does H+ increase?

thx so much
9. (Original post by itsConnor_)
why does H+ increase?

thx so much
The half equations are:
1/2 Cl2 + e- <=> Cl- which has the e.m.f value +1.36 V
2H+ + 2e- <=> H2 which has the e.m.f value 0.00 V
The Cl2 half cell has the most positive (read less negative) value and therefore remains as written. As the hydrogen half cell is more negative its equilibrium is reversed and is now H2 <=> 2H+ +2e-
As the hydrogen half cell has 2 electrons on the right hand side of the equilibrium but the chlorine half cell only has one you have to double all components in this equilibrium making it
Cl2 + 2e- <=> 2Cl-
simply add them together and cancel the electrons to get the overall cell reaction
Cl2 +H2 <=> 2H+ + 2Cl-
In the forward reaction both more Cl- ions and H+ ions are being produced.
pH is just -log(concentration of hydrogen ions) meaning a lower pH = a higher H+ concentration

Hope this helps
10. (Original post by VMD100)
The half equations are:
1/2 Cl2 + e- <=> Cl- which has the e.m.f value +1.36 V
2H+ + 2e- <=> H2 which has the e.m.f value 0.00 V
The Cl2 half cell has the most positive (read less negative) value and therefore remains as written. As the hydrogen half cell is more negative its equilibrium is reversed and is now H2 <=> 2H+ +2e-
As the hydrogen half cell has 2 electrons on the right hand side of the equilibrium but the chlorine half cell only has one you have to double all components in this equilibrium making it
Cl2 + 2e- <=> 2Cl-
simply add them together and cancel the electrons to get the overall cell reaction
Cl2 +H2 <=> 2H+ + 2Cl-
In the forward reaction both more Cl- ions and H+ ions are being produced.
pH is just -log(concentration of hydrogen ions) meaning a lower pH = a higher H+ concentration

Hope this helps
Got it, thanks! So just look at your overall equation which i got and you can see that H+ is being produced, so conc. of H+ increases and hence pH decreases
11. (Original post by tcameron)
no they're different
But in your calculations you used gdm-3 where the number of moles would be used? Can you elaborate please?? Sorry
12. (Original post by Dentistry101)
But in your calculations you used gdm-3 where the number of moles would be used? Can you elaborate please?? Sorry
that's because the question wanted the answer as gdm-3 otherwise I'd do it as normal in moldm-3 which is more simpler
13. (Original post by Serine Soul)
It's good to challenge yourself, you never know when OCR will chuck in a crappy question (cough cough 'magic tang'
I just did that paper!! How did they expect us to know to choose lactic acid?!? Thanks
14. (Original post by GawdHeresEmmie)
I just did that paper!! How did they expect us to know to choose lactic acid?!? Thanks
Absolutely no idea..

But from there, I had learnt that pKa is round about the same as pH, so you'd go for the acid with the closest pKa value to the target pH
15. (Original post by GawdHeresEmmie)
I just did that paper!! How did they expect us to know to choose lactic acid?!? Thanks
The pKa of lactic acid is closest to the desired pH.
16. (Original post by tcameron)
because they are the products -
the products originally was a 0 moles whilst CH4 initial moles was already something at the start of the reaction
So you would directly compare the mole ratios of the products as they started off as the same intial moles of 0 rather than the moles of the reactant, so 0.168x3 in the 1:3 ratio would be 0.504 moles for H2
Hope that sort of makes sense
no
17. (Original post by TeachChemistry)
The pKa of lactic acid is closest to the desired pH.
I wish I had known this earlier... I bet I spent a good 10 minutes re-reading the question trying to figure out where you got the information revealing which acid to use, Thank you so much!
18. (Original post by RayMasterio)
no
whenever we do kc calculations the inital moles of the reactants is always known as that's what is added to make the products so the initial moles of the products is always 0
so that's why you needed to use the products as their initial moles were 0
all it was was 0.168x3
19. Are we supposed to remember the pKa for Congo red, Methyl red, thymol blue and thymolphthalein? and their colour changes?
20. (Original post by TeachChemistry)
The pKa of lactic acid is closest to the desired pH.
That means that the ka value of the weak acid is close to the H+ concentration, right?
Meaning that the ratio of the weak acid to the conjugate base is close to 1, why does a buffer function best when the ratio is close to 1?

Posted from TSR Mobile

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: September 26, 2016
Today on TSR

### Loughborough better than Cambridge

Loughborough at number one

### Can I date a girl with no boobs?

Poll
Useful resources

Can you help? Study Help unanswered threadsStudy help rules and posting guidelines

## Groups associated with this forum:

View associated groups

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE