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 -----------------------------------------------------------------
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
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?!
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!!!
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?
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
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!!
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
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