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

1. Any help with this?
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2. (Original post by tcameron)
Any help with this?
nN=m/Mr = 63.64/14 (Take % as being the mass because it is a ratio.) = 4.55mol nO=m/Mr = 100-63.64/16 = 36.36/16 = 2.27 mol

Hence, nN:nO = 2:1

Hence N2O.
3. (Original post by Timon512)
nN=m/Mr = 63.64/14 (Take % as being the mass because it is a ratio.) = 4.55mol nO=m/Mr = 100-63.64/16 = 36.36/16 = 2.27 mol

Hence, nN:nO = 2:1

Hence N2O.
I got that but the question also said about the density being 1.8333gdm-3 so would there need to be a conversion to find the Mr because I don't think N2O is the right answer

don't worry I've figured it out now, you had to x by 24 to find the Mr
4. (Original post by tcameron)
I got that but the question also said about the density being 1.8333gdm-3 so would there need to be a conversion to find the Mr because I don't think N2O is the right answer

don't worry I've figured it out now, you had to x by 24 to find the Mr
Multiply what by 24? Could you write your working out please? Why did you use V/24=N?
5. I can't believe how badly I've been taught redox and fuel cells

I need to go over it 100 times
6. (Original post by sunsri101)
But you do want electrons to move between the half cells. Current is the flow of charged particles, so if you want electrons to flow, surely you do want current?
And the solution concentrations do change over time, that's why batteries run out unless they are rechargeable- right?

Yes batteries run down because current has been allowed to flow.

Think about what you are measuring (emf) as a push from the cell. You need to measure the magnitude of that push by, in effect, pushing back using electrical resistance. If you resist enough, current will be negligible.

If you are ok with physics think about the resistance that an ammeter must have, because it goes in series to the other components, and the resistance a voltmeter must have because it goes parallel to the component you are measuring.

I will draw a picture and upload it.
7. Why is the conc of butanoic acid 0.1?

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8. (Original post by Serine Soul)
I can't believe how badly I've been taught redox and fuel cells

I need to go over it 100 times
Students find this hard because teachers do not explain the difference between voltaic and electrolytic cells.

http://chemwiki.ucdavis.edu/Core/Ana...s/Electrolysis
9. (Original post by itsConnor_)
Why is the conc of butanoic acid 0.1?

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the butanoic acid is in excess (0.0125 mol), whereas NaOH is 0.0025 mol.

So, moles of butanoic acid remaining= 0.0125-0.0025=0.01

conc of butanoic acid= 0.01/0.1=0.1 mol dm^-3
10. (Original post by TeachChemistry)
Yes batteries run down because current has been allowed to flow.

Think about what you are measuring (emf) as a push from the cell. You need to measure the magnitude of that push by, in effect, pushing back using electrical resistance. If you resist enough, current will be negligible.

If you are ok with physics think about the resistance that an ammeter must have, because it goes in series to the other components, and the resistance a voltmeter must have because it goes parallel to the component you are measuring.

I will draw a picture and upload it.
11. (Original post by itsConnor_)
Why is the conc of butanoic acid 0.1?

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Basically you find the moles of butanoic acid originally and the moles of Naoh. The moles of butanoic acid in equilibrium are the moles of butanoic acid you calculated minus the moles of Naoh you calculated. Then find the concentration of butanoic acid doing c x v= n using the new moles you calculated and you get 0.1
12. (Original post by TeachChemistry)
Hey, I tried to solve a really difficult redox titration question on this thread- but couldn't seem to be able to - Can you help please??

http://www.thestudentroom.co.uk/show...php?p=65895832
13. Do I need to be doing any specific ABG/CER revision before EEE- do they ever ask random AS questions?
14. (Original post by thesmallman)
the butanoic acid is in excess (0.0125 mol), whereas NaOH is 0.0025 mol.

So, moles of butanoic acid remaining= 0.0125-0.0025=0.01

conc of butanoic acid= 0.01/0.1=0.1 mol dm^-3
(Original post by Dentistry101)

Basically you find the moles of butanoic acid originally and the moles of Naoh. The moles of butanoic acid in equilibrium are the moles of butanoic acid you calculated minus the moles of Naoh you calculated. Then find the concentration of butanoic acid doing c x v= n using the new moles you calculated and you get 0.1
Thanks guys

Gonna try an Edexcel one I found and see if I can do it this time!
15. (Original post by thesmallman)
Hey, I tried to solve a really difficult redox titration question on this thread- but couldn't seem to be able to - Can you help please??

http://www.thestudentroom.co.uk/show...php?p=65895832
Your chemical equations are correct. If you work forward from a supposedly correct answer of 0.51M you will find that the titre values for both oxidizing agents are nearly the same value so there is something wrong in the question.

And I think I might have just spotted the error in the question. BRB.
16. (Original post by Serine Soul)
I can't believe how badly I've been taught redox and fuel cells

I need to go over it 100 times
Same!! We basically had to teach ourselves it and i'm still pretty inconsistent on the exam qs

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17. (Original post by thesmallman)
Hey, I tried to solve a really difficult redox titration question on this thread- but couldn't seem to be able to - Can you help please??

http://www.thestudentroom.co.uk/show...php?p=65895832
If you get the question number and the paper and put it into youtube I think MaChemGuy did a really clear explanation it
18. Could anyone help with june 14, with the laat part of the last question?

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19. (Original post by Dentistry101)
Multiply what by 24? Could you write your working out please? Why did you use V/24=N?

So basically you find the emperical formula as normal as N2O
because gdm-3 x24 = gmol-1 which is the Mr to confirm that the molecular mass is N2O as the answer
x24 because number of moles = v/24 for a gas, you just have to make the association with density and how much a gas occupies a space
20. How does the hydrogen fuel cell actually work?
I don't understand all the stuff about in alkaline or acidic conditions

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