Competition - post a photo you've taken of nature >>

Equilibria, Energetics and Elements (F325) - June 2011 Exam.

Scroll to see replies

At the end of the day, it's in the past now, you've just got to prevent it from having a detriment to your other exams. Jan 2011 paper was 61% for an A, I thought that was a harder paper than the Jan one personally, but we shall see!

Reply 2521

Rosi M

Original post by LilacTweet

I got this, and a lot of people I know got this. I would've got 76% except going back to the orginal equation in the first step there were half the number of moles in that. so I divided my mole by two and got a smaller mass.... however my chemistry is poor so I have a feeling 67ish% was right....hopefully we'll just lose one mark for that mistake though? :/

Yep hopefully got a few marks there somewhere. Im sure i got atleast 1 or 2 of the equations wrong.. they were wierd!
Didnt times by 2 for the hydration thing so lost it there too :frown:

Thought it went ok, but not anymore

Reply 2522

Rosi M

Original post by haydyb123

How did you find it?! Omg i rememberd what you taught me about the strong and weak acid for acid base pairs.. that the strong acid will lose the H! :smile:

Reply 2523

PhilliChilli

so ......F324 24th june....uh oh

Reply 2524

jam.wa

I've always been a Starmix man myself anyway.

This was a hard paper but at the end of the day it was all on the spec or obviously strech and challenge, so yea hope for low grade boundaries but to say its on the same level as the bio was is just not true..

Reply 2527

BA1

Magic tang was in fact quite easy. Choose the acid with a pKa value closest to the desired pH (so lactic acid).

Ka = \frac{[H^+][A-]}{HA}

Rearrange to get the ratio of acid to salt:

\frac{[HA]}{[A-]} = \frac{[H^+]}{Ka}

Where:

[H^+] = 10^{-pH}

And:

Ka = 10^{-pKa}

of the chosen acid.

The ratio was 2.02 : 1 if you chose lactic acid, so the concentration of the acid had to be approximately double that of its salt.

Reply 2528

susan23

Original post by clemencyb

i heard back saying that if a significant proportion of the cohort complains a review of the paper will be undertaken.Everyone complain pls...there isnt the material available to prepare and the lack of timing skews results in favour of those with extra time - as it was impossible to finish

[email protected]

http://www.ofqual.gov.uk/

PLEASE COMPLAIn guys!!!!

Reply 2529

LilacTweet

Could anyone go through the hydration one? I got confused when there was another arrow under the lattice enthalpy.....

Reply 2530

CoventryCity

Original post by BA1

Magic tang was in fact quite easy. Choose the acid with a pKa value closest to the desired pH (so lactic acid).

Ka = \frac{[H^+][A-]}{HA}

Rearrange to get the ratio of acid to salt:

\frac{[HA]}{[A-]} = \frac{[H^+]}{Ka}

Where:

[H^+] = 10^{-pH}

And:

Ka = 10^{-pKa}

of the chosen acid.

The ratio was 2.02 : 1 if you chose lactic acid, so the concentration of the acid had to be approximately double that of its salt.

Yep just realised that about 10 minutes ago, I left this question to last so I rushed it in a few mins so didn't think it through properly.
After you have your ratios is that all you had to?

Reply 2531

pan_93

is anyone putting up the paper...i thought someone had it

Reply 2532

Intollerant

Stop complaining! It was hard YES. But it was hard for EVERYONE and it challenged people of all abilities. Atleast it was fair, unlike the biology exam

Reply 2533

haydyb123

Original post by Rosi M

How did you find it?! Omg i rememberd what you taught me about the strong and weak acid for acid base pairs.. that the strong acid will lose the H! :smile:

I'm glad, that means my help was worthwhile which consoles me! :biggrin:

On the subject of the exam, I was stumped by the Buffers or infamous 'Magic Tang' question, I thought we were missing a piece of information. Got the last answer by complete fluke as 67.5%... messed up the Gibbs free energy, i found T delta S and then just proceeded not to find delta S from this and just substituted T delta S into the equation...

But other than that, I have no idea how i've done! Needed 86% for an A overall, doubt I got that.

Reply 2534

yu80359

PLS~~~~~~~~~~~~~~~~~~~~~~~~~ Question paper someone I really need it

OHNO!CR

I did benzene for the buffers one said add in sodium benzanoate to generate the conjugate base c6h5c00-. and then worked out the ratio to be 0.23 using. [H+]= Ka * [C6H5COO] / [C6H5COONa] ? maybe 3 or 4 marks perhaps?

Reply 2537

BA1

Original post by CoventryCity

Yep just realised that about 10 minutes ago, I left this question to last so I rushed it in a few mins so didn't think it through properly.
After you have your ratios is that all you had to?