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Is the fingerprint region below 1500 cm-1. If so i think I just made one mistake -- the petan-3-one one, just had a mental block. Hopefully 85+ UMS
Originally Posted by dups45
Ok, i did it differently to you

Find the no moles acid
Find the no moles NaOH added

[H+] = sqrt Ka x no moles in excess

-log10[H+]

Im starting to think its wrong now though
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Tht method is only valid if nothing has been added to the weak acid.
The [H+]^2/HA approximation only works if nothing has been added to the weak acid. Hope that helps
Reply 62
dups45
Ok, i did it differently to you

Find the no moles acid
Find the no moles NaOH added

[H+] = sqrt Ka x no moles in excess

-log10[H+]

Im starting to think its wrong now though

Pretty sure that's what I did too. It's the way we have been taught it.



edit: sorry, think I am getting confused over which question we are talking about.
Well I'm not sure because half the people here think its 4.08 and the others 2.22 or something. Guess we'll have to wait and see.
Greenmile
edit: sorry, think I am getting confused over which question we are talking about.


We are talking about the second part to the buffer question. In the first you had to calculate pH of acid using KA. Then in second you had to calculate pH where acidic buffer is made.
Hmm...I used the Ka equation too for the buffer question but for some reason I divided the moles of NaOH and CH3CH2COOH by the total volume (40cm3). Is this completely wrong?!
Reply 66
did anybody actually divide the acid and salt by their volumes or is it just me???
Volume doesnt come into the equation since the mole and concentraion ratios are in the same overall volume.
Reply 68
oh and this was probably the most strangest paper ive done...god Jan07 paper was so much easier....section B was good as ever...but Kp my goodness and those stupid curves which arent even the major its on the specs...grrrrrrrr!!!
Reply 69
oh dolly...how many marks was it out of??...5?...id lose 2 then i think
Yeah, thats what Im thinking as well! I divided the moles by the total volume which was 40, because you needed their concentration to put it into the equation.

Also for the esterification bit with the amino acid, is it acceptable to put
COO-Na+ rather than CooNa?
Reply 71
i think they will accept that...and o well about the other bit lol..:frown:
Reply 72
Yeah i got 4.08 for the buffer question. I think most ppl forgot to divide by 40 e.g. Ka x no of mole of propanoic acid/40
CH3CH2COO/40

I think i got basically full marks on section B, dont think i got anything wrong so if i gt around 45-50/60 on section A i should be OK
Grrr, its really annoying because I know I slipped up loads. I've definitely lost about 10 marks, but I'm sure there's more :frown:

What equation did everyone write for the nitration of benzene? And how did you name the amide? I put n-methylpropanamide because all I could think of was n-phenylethanamide!!
Reply 74
Argh, im trying to revise biology now, but i keep on coming back on here and checking my answers. I really do not know if i got the Buffer calculation right or not. I divided the number of moles CH3CH2COOH by the total volume aswell to get the concentration

[H+] = sqrt Ka x [CH3CH2COOH]
Reply 75
for the reduction of nitro compound to phenylamine was it 3[H] or 6??
I put 4H because the two NO2 groups were becoming two NH2 groups
I got 4.08 for the buffer and did it by:

[H+] = (Ka x [CH3CH2COOH[)/[CH3CH2COO-]

[CH3CH2COOH-] is just the concentration of NaOH.

You don't even need to find the concentrations as the volumes cancel out in the equation, I checked it and got the same answer with the number of moles and concentrations.

And was it dehydration or elimination in the type of mechanism question?

Think I did quite well, completely failed on the total pressure question, got like 600 or something O_O
I put 12[H] because you end up with 4H20 so need more. I think thats right :s-smilie:
Reply 79
yup i put 12[H] aswell..and yep i also keep coming on here to check my answers lol when i should be revising for my bio's 2mro! :frown:

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