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OCR B Salters - F335 Exam - 15 June 2011

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Original post by Willetts.Josh
Hey, I think I remember what it was.

The value of Ka was 6.2x10^-8
And they used a concentration of 0.1 of A- (I can't remember the formula)
And you knew the pH was 7, so the conc of H+ ions was 10^-7

6.2x10^-8 = (10^-7 x 0.1)/[HA]

So the concentration of HA was 5/31 moldm-3

You only wanted 1dm^3 so there was 5/31 mols of it needed.
The Mr was 120, so 5/31 times 120 makes roughly 19.4g


Exactly what I got / did :smile:
Original post by arsenalman
This chemistry exam is the first exam ive been happy after this summer. My other exams ive done were S2 and unit 4 biology both by edexcel. I think edexcel have made all their exams harder or different from the past papers this summer or is it just me?


I've only done Chemistry and C4 MEI on Monday. But they seem different, yeah.
Some of my friends took Edexcel Biology Unit 4, and said that it was almost like they forgot to put in proper biology? :biggrin: I suppose we'll find out next week with unit 5!
Reply 842
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04ayasin
Original post by Willetts.Josh
Hey, I think I remember what it was.

The value of Ka was 6.2x10^-8
And they used a concentration of 0.1 of A- (I can't remember the formula)
And you knew the pH was 7, so the conc of H+ ions was 10^-7

6.2x10^-8 = (10^-7 x 0.1)/[HA]

So the concentration of HA was 5/31 moldm-3

You only wanted 1dm^3 so there was 5/31 mols of it needed.
The Mr was 120, so 5/31 times 120 makes roughly 19.4g


Yep thats what I did cant remember the answer I wrote down, but thats definately how I worked it out.
Original post by MyJunkIsYou
What did people put for the oxidation and reduction question? Completely guessed that!

On the enthalpy diagram should the line for the aqueous ions have been above or below the line for the solid lattice?

Also the question where you had to do a 2 step process to get to a carboxylic acid and draw the intermediate? I couldn't remember what it was so I used the reactions given in the data sheet adding a CN to make an intermediate that was a nitrile and then refluxing with H+/H2O to get a carboxylic acid. Anybody know if I'll get any marks for that?


There was a similar question to this I did a few days ago in an old past paper. In step one, hydrogen cyanide plus NaOH will replace the two bromines with OH groups. You then reflux this with acidified potassium dichromate(VI) to get the carboxylic acid group.

The NMR question made me soo unhappy.
Original post by brendan.
I done this too.. in the assumption that the COOH donated the proton to the amine..


Yep I circled the COOH group because it was the only part of the molecule all three molecules had in common.
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goodezy12
Original post by microfatcat
Yep I circled the COOH group because it was the only part of the molecule all three molecules had in common.


Although, they did all have a ch2 group before that as well..
Original post by goodezy12
Although, they did all have a ch2 group before that as well..


Yeah, I think I circled the carboxylic acid group, including the CH2 group that was just before it.
Reply 847
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goodezy12
Original post by brendan.
Yeah, I think I circled the carboxylic acid group, including the CH2 group that was just before it.


I did that, but really messily as changed it just before the end... hopefully I get given it. Considering I only need 77/120 for this paper, I've messed up ridiculously so far. :/
Original post by goodezy12
I did that, but really messily as changed it just before the end... hopefully I get given it. Considering I only need 77/120 for this paper, I've messed up ridiculously so far. :/


Ah don't give up hope! You may have actually done better than you thought you done, or they could give you the benefit of the doubt in some of your answers..
If the general response is that it was a relatively hard paper, then maybe the grade boundaries will come down. Also if it's anything like January, an A was like 80/120 or something in Raw Marks, so UMS would be higher.
Reply 849
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DannyM786
Well that was a lovely birthday surprise...having the most difficult chemistry paper I've ever seen :-( felt so prepared for that as well
Original post by Tetanus
Were they not able to dissolve in one another because they could form permanent dipole-permanent dipole, but could not dissolve in water because they can't form Hydrogen Bonds. I put something like that + loooooads of waffle.


I put this!


Original post by cleverbong
I put gas liquid chromatography.


Yup put that too.

Original post by Tetanus
Wasn't it just 1,3 diaminobenzene? :s-smilie:


Yup
Original post by arsenalman
Wasnt the bond angle on the SF6 109 degrees?


90 wasn't it?
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Tetanus
Original post by DannyM786
Well that was a lovely birthday surprise...having the most difficult chemistry paper I've ever seen :-( felt so prepared for that as well


:birthday: for yesterday!
Reply 853
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Tetanus
Original post by limetang
90 wasn't it?


It was 90. I put 109.5 though, just for da lulz.
Original post by limetang
90 wasn't it?


There were two angles you could've labelled.
Original post by tom108
Im really confused. Didnt reductiction also occur is fluroine? Went from -1 to -4 ????????????


I put sulphur was oxidised and fluorine was reduced.


errrr why was I negged for this...?
(edited 12 years ago)
if ocr put f334 before f335 i reckon i would have done better because of the synoptic element. :/
Original post by microfatcat
I put sulphur was oxidised and fluorine was reduced.


I put copper and sulphur down (can't remember which I said was oxidised or reduced now though). I was probably wrong though =P.
(edited 12 years ago)
Original post by microfatcat
There were two angles you could've labelled.


It had 6 fluorines around it. That made it octahedral (I think), and I thought all the bond angles in an octahedral molecule were the same.
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SarahTM
12
Original post by patrick93
if ocr put f334 before f335 i reckon i would have done better because of the synoptic element. :/


I thought this. OCR suck spectacularly this year.

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