The Student Room Group
Uni students - Complete our survey >>
Uni students - Complete our survey >>
Uni students - Complete our survey >>

ocr a f325 revision thread

Scroll to see replies

Reply 1520
Avatar for sapna09
sapna09
Anyone have any tips on unstructured titrations?! Honestly can't do them and they are usually worth 7 marks! Thanks


Posted from TSR Mobile
Original post by fayled
That is what I wrote :s-smilie:

oops, lol what a silly billy
Concentrated nitric acid is a powerful oxidising agent. Concentrated nitric acid oxidises
sulphur to sulphuric acid. Nitrogen dioxide and another product are also formed.
(i) Suggest a balanced equation for this reaction.

For this question i got the correct reactant and product but the balancing is hard, how would you do it?

HNO3 + S ---> H2SO4 + NO2 + H20
Reply 1523
Avatar for fayled
fayled
9
Original post by otrivine
Concentrated nitric acid is a powerful oxidising agent. Concentrated nitric acid oxidises
sulphur to sulphuric acid. Nitrogen dioxide and another product are also formed.
(i) Suggest a balanced equation for this reaction.

For this question i got the correct reactant and product but the balancing is hard, how would you do it?

HNO3 + S ---> H2SO4 + NO2 + H20


The water isn't needed, then you can easily balance...
Original post by fayled
The water isn't needed, then you can easily balance...


But mark scheme accepts either hydrogen or h2o
Original post by otrivine
But mark scheme accepts either hydrogen or h2o

How many marks is it worth?
Reply 1526
Avatar for Pride
Pride
21
Original post by sapna09
Anyone have any tips on unstructured titrations?! Honestly can't do them and they are usually worth 7 marks! Thanks


Posted from TSR Mobile


I think apart from the obvious advice of practice lots of questions, I would recommend you just think, what do I definitely know? And build it up from there.

Don't expect to always be able to just look at the question and know exactly what to do. Sometimes when it's unstructured, you just have a punt, you just use what you have been given. Then you work out what you can do next.

Normally for example the first step is to work out the number of moles in the average titre they give you. Take it a step at a time then. You might be able to work out a ratio using an equation, and then go on from there.

Got any specific questions you'd like help with?
Original post by kitkat95
How many marks is it worth?


1 mark
Original post by otrivine
1 mark


Well I got: 6HNO3 + S ---> H2SO4 + 6NO2 +2H2O
I created two half equations: HNO3 + H+ + e ---> NO2 + H2O
S + 4H2O ---> H2SO4 + 6H+ + 6e
Then multiplied the HNO3 equation by 6:
6HNO3 +6H+ + 6e ---> 6NO2 + 6H2O
And combined the two equations. But that seems like waaay too much work to do for just one mark so not sure if this is right? I think the final answer is right but there must be a quicker way to do it.
Original post by kitkat95
Well I got: 6HNO3 + S ---> H2SO4 + 6NO2 +2H2O
I created two half equations: HNO3 + H+ + e ---> NO2 + H2O
S + 4H2O ---> H2SO4 + 6H+ + 6e
Then multiplied the HNO3 equation by 6:
6HNO3 +6H+ + 6e ---> 6NO2 + 6H2O
And combined the two equations. But that seems like waaay too much work to do for just one mark so not sure if this is right? I think the final answer is right but there must be a quicker way to do it.


I know there is a method involving oxidation numbers but how
Original post by otrivine
I know there is a method involving oxidation numbers but how


The N in HNO3 has an oxidation number of 5 (oxygen is 2- and hydrogen 1+ so (3 x -2)+1= -5 so N=5). And the oxidation number of the N in NO2 is +4 because oxygen is 2- so 2 x -2 = -4 so to balance it out N must be +4.
This means that the N has to gain an electron (be reduced) so:
HNO3 + e --> NO2
Then the oxygen atoms have to balance out so a H2O is added to the products: HNO3 + e ---> NO2 + H2O. Then the hydrogen atoms need to balance so a H+ is added o the reactants to get:
HNO3 + H+ + e --> NO2 + H2O
Then do the same process for the S.
Reply 1531
Avatar for narli
narli
i hate this exammmmmmmmmmmmmmmmmm
Reply 1532
Avatar for Namod
Namod
10
Ok, don't forget to finish them.
Any chemistry genius can answer those questions?
january 2013 paper F325
Q1(v) - How to get Cr?
Q6(a) - The mark schemes says enclosed container what do they mean by that?
Q7(b)(ii) - How to get these equations?
Q7(c)(ii) - How to do that?
Q8(c)(ii) - How do we that?

June 2012
3(c)(ii) - How to do that?

Jan 2011:
7(a)(i) - How to get moles of O2

June 2010:
6(e) - How do I find C?
7(b) - Why in the mark schemes they times the concentration of H2O2 by 34
anyone want to explain part 3e on Jan 11 ? I know its 2 marks but I just don't understand it :frown:
Also with the dicarboxylate on the previous page how did you know the diagram? I haven't come across it figured its a circular as of the cyclo but just messed up the O groups -didn't realise there were 2 double bonds :frown:
Reply 1534
Avatar for Better
Better
8
Re-doing Past Papers is making me feel really good.

Thanks again whoever uploaded that video on Titrations! I really just didn't understand the AS Theory Well Enough before but now I do!

My Current Marks are (isolated under timed conditions - Funtry how do you get through the papers so fast - it still takes me about 1hr45)
Specimen Paper: 86/100 (re-done - Thank you Funtry for Clearing up Errors)
June 2010: 91/100 (re-done)
Jan 2011: 86/100 (First time round, not re-doing)

Then I've got June-11 to mark which I did yesterday

Then today I'm re-doing Jan 12, Jun 12, and doing some Chem 4 and Physics 4/5 Prep!!!
Original post by MedMed12
anyone want to explain part 3e on Jan 11 ? I know its 2 marks but I just don't understand it :frown:
Also with the dicarboxylate on the previous page how did you know the diagram? I haven't come across it figured its a circular as of the cyclo but just messed up the O groups -didn't realise there were 2 double bonds :frown:



Yes that was a nasty question, literally you have to apply your knowledge regarding the cis and trans platin, but instead of the simple NH3 and Cl , they want you to put a dicarboxylate ion
Original post by otrivine
Yes that was a nasty question, literally you have to apply your knowledge regarding the cis and trans platin, but instead of the simple NH3 and Cl , they want you to put a dicarboxylate ion


oh no I figured that :tongue: I just meant I didnt know how to work out the structure of dicarboxylate so tried to intelligently guess (didnt know there were double bonds)
Reply 1537
Avatar for fayled
fayled
9
Original post by Namod
Ok, don't forget to finish them.
Any chemistry genius can answer those questions?
january 2013 paper F325
Q1(v) - How to get Cr?
Q6(a) - The mark schemes says enclosed container what do they mean by that?
Q7(b)(ii) - How to get these equations?
Q7(c)(ii) - How to do that?
Q8(c)(ii) - How do we that?

June 2012
3(c)(ii) - How to do that?

Jan 2011:
7(a)(i) - How to get moles of O2

June 2010:
6(e) - How do I find C?
7(b) - Why in the mark schemes they times the concentration of H2O2 by 34


For the first on your list, you are given the average mass of one atom. Now if you multiply this by the Avogadro constant you will get the average mass of one mole (i.e the molas mass). Then use the periodic table.

For the second, the hydrogen gas has to be put in through a container otherwise it would escape. Like this:
http://www.4college.co.uk/a/ss/cell6.gif
(edited 10 years ago)
Original post by MedMed12
oh no I figured that :tongue: I just meant I didnt know how to work out the structure of dicarboxylate so tried to intelligently guess (didnt know there were double bonds)


I think the structure would have been similar to the ethanoate ion?


Who neg rep you?
Original post by otrivine
I think the structure would have been similar to the ethanoate ion?


Who neg rep you?


hmm thats a true point! I guess it one of the A*/synoptic questions seeing as its only 2 marks but I because I got the structure wrong I wasn't sure if I'd lose the second mark when drawing it in replacement of cl- in the platin but I took it away just incase/

I don't know :frown: meh nevermind

Quick Reply

Related discussions

Latest

Trending

Trending

Articles for you