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# ocr a f325 revision thread watch

1. (Original post by Sakib)
How would you answer this question: This question looks at two acids: methanoic acid, HCOOH, a weak organic acid; nitric acid, HNO3, a strong acid which can also act as powerful oxidising agent. d) Nitric acid, HNO3, is sold by a chemical supplier as a 65% solution, by mass. As supplied, each cubic decimetre of this nitric acid has a mass of 1400 g. Calculate the pH of this solution.

Usually I would use the concentration of HNO3 to workout the answer, ie, pH= -log(H+), but we are not provided with the concentration or anyway to work it out.
You can work out the moles of the HNO3 cant you?
2. (Original post by zef1995)
I'm sorry for the delay in response!

I think I've got it, so I'll try and explain what I've done, and if there's anything you're unsure of/disagree with, then feel free to say so

Okay...so you have 25 cm3 ammonium vanadate with concentration 0.100 mol dm-3. From this, we can calculate the moles of ammonium vanadate:

0.100 x 25/1000 = 2.5 x 10-3 moles VO3-

We also know that 30 cm3 of 0.0500 mol dm-3 MnO4- is required to oxidise V2+ back to VO3- in the titration.

Calculating the moles of MnO4- used: 0.500 x 30/1000 = 1.5 x 10-3 moles MnO4-

We can now calculate a ratio of MnO4- : VO3-

1.5 x 10-3 : 2.5 x 10-3

1:5/3 = 3:5 ratio.

Looking at the half equation for MnO40-, we now know 3 MnO4- react for every 3 VO3-.

3MnO4- + 24H+ + 15e- -> 3Mn2+ + 12H2O

There's a transfer of 15e-, so 15e- will be produced by 5 moles V2+ (3:5 ratio again).

15/5 = 3

So there's a transfer of 3 electrons - MnO4- gains them in this reaction, so V2+ loses them. As it loses 3 e-, it is oxidised for V2+ to VO3- (+5 oxidation state).

I hope I've explained it clearly, and if you have any questions please feel free to ask, it's not a problem

I couldn't do the very last question where you have to write out the full redox equation. How did u get on with that? It would be great to get a good answer from you
3. (Original post by Gulzar)

I couldn't do the very last question where you have to write out the full redox equation. How did u get on with that? It would be great to get a good answer from you
Thank you very much, I tried to explain it as clearly as I could - I'm glad it makes sense!

Was this the January 2013 paper? I didn't get it right when I did the mock, but I understand why I didn't now and I can do it if you'd like me to explain it
4. Just got 93/100 in the Jan'11 paper, happy. Just gonna double check to make sure I wasn't too easy on the marking
5. Yep, just remarked, lost 3 marks. Still, it's ~ full UMS
6. (Original post by zef1995)
Thank you very much, I tried to explain it as clearly as I could - I'm glad it makes sense!

Was this the January 2013 paper? I didn't get it right when I did the mock, but I understand why I didn't now and I can do it if you'd like me to explain it
You've already explained it very well
It's just that very last bit of that exact question on the jan 2013. It would be great if you could explain that bit when you have the time
7. (Original post by Gulzar)
You've already explained it very well
It's just that very last bit of that exact question on the jan 2013. It would be great if you could explain that bit when you have the time
Aw thank you, I'm glad I did!
Okay no problem at all! If you don't mind, I don't think I'll be able to do it tonight (it's likely I'll get it wrong/miss something out because my brain is going to sleep!) :P
However, I will be more than happy to go through it tomorrow, if that's not a problem
8. (Original post by otrivine)
Perfect=100%
On a different note OCR Chemistry is surprisingly very challenging. I got 86% on Jan 2011 but I think that may have been an easy paper.

I just did June 2011 in exactly 2 hours and it had a bunch of BrainTeasers. ARGH. God knows how people finished it in 1hr45 mins before.

I will be posting up a few questions about my confusion PLEASE can one of you Chemistry Gods help me.
9. (Original post by Funtry)
Just got 93/100 in the Jan'11 paper, happy. Just gonna double check to make sure I wasn't too easy on the marking
Wow, nice one! That's amazing, even if you lost 3 marks, that's still 90 out of 100 D:
Congratulations again, I hope the real thing goes just as well for you
10. (Original post by zef1995)
Wow, nice one! That's amazing, even if you lost 3 marks, that's still 90 out of 100 D:
Congratulations again, I hope the real thing goes just as well for you
Hi hows revision going
11. (Original post by otrivine)
Hi hows revision going
Hi, it's going well thank you! Managed to get a bit more revision in today
12. (Original post by zef1995)
Hi, it's going well thank you! Managed to get a bit more revision in today

No worries, its going okay only want to do legacy papers now !

Today had 6 hours of revision at college , 3 hours of chem and covered equilibrium kinetics so much
13. (Original post by Funtry)
Just got 93/100 in the Jan'11 paper, happy. Just gonna double check to make sure I wasn't too easy on the marking
I only got 86/100 did it like 2 days ago now but marked it harshly. I'm personally setting myself 94 Marks + for 100 UMS for all papers I do, just in case

Have you done June 2011 yet? I'm going to mark it now, but it was very very tough compared to Jan 2011.

Can anyone care to Explain? REP if you can

JANUARY 2011: Question 3E - The Examiners reports said virtually no one could do this question. Only 2 Marks the question with R-S-S-R
14. (Original post by Better)
I only got 86/100 did it like 2 days ago now but marked it harshly. I'm personally setting myself 94 Marks + for 100 UMS for all papers I do, just in case

Have you done June 2011 yet? I'm going to mark it now, but it was very very tough compared to Jan 2011.

Can anyone care to Explain? REP if you can

JANUARY 2011: Question 3E - The Examiners reports said virtually no one could do this question. Only 2 Marks the question with R-S-S-R
Only done Jan11

I got the second half, not the first half though.
15. (Original post by Funtry)
Yep, just remarked, lost 3 marks. Still, it's ~ full UMS
Please can I have some tips on the sources that you use for revision apart from a text book and past papers.
16. (Original post by überambitious_ox)
Please can I have some tips on the sources that you use for revision apart from a text book and past papers.
I use Chemguide, Knockhardy and These questions primarily, as well as my own notes.
17. (Original post by otrivine)
No worries, its going okay only want to do legacy papers now !

Today had 6 hours of revision at college , 3 hours of chem and covered equilibrium kinetics so much
That sounds good, there are plenty of legacy papers around

Wow, lots of revision, at least it was productive
18. (Original post by Gulzar)
You've already explained it very well
It's just that very last bit of that exact question on the jan 2013. It would be great if you could explain that bit when you have the time
Do you happen to have the answer for the last part of the last question on January 13 (the equation)?
I just wanted to check whether I was right before I start explaining the wrong equation to you!
I wrote the answer down when my teacher explained it in class, but of course...I've lost the bit of paper with it on. Organisation at its best!
19. (Original post by zef1995)
That sounds good, there are plenty of legacy papers around

Wow, lots of revision, at least it was productive
Exactly!

you know the legacy papers right , are they the ones on the OCR website?
20. (Original post by zef1995)
Do you happen to have the answer for the last part of the last question on January 13 (the equation)?
I just wanted to check whether I was right before I start explaining the wrong equation to you!
I wrote the answer down when my teacher explained it in class, but of course...I've lost the bit of paper with it on. Organisation at its best!
Sure
I've got it down as 5V2+ + 3MnO4- + 3H2O ------> 5VO3- + 3Mn2+ + 6H+

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