ocr a f325 revision thread

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.

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

That is a brilliant answer

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

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

Perfect=100%

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

Hi hows revision going

Hi, it's going well thank you! Managed to get a bit more revision in today
How about you?

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

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.

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

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

That sounds good, there are plenty of legacy papers around

Wow, lots of revision, at least it was productive

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!