LSOP revision...guys from sop help!

heya guys am stuck on the 1st and last questions from the electrochemistry booklets....please help as i have no idea where to start tese...i attach a copy of the questions!
thank u!

dude, totally feel sorry for you. my friend in 2nd year said to me that they're gonna have 150mcqs in 1 hour...that literally sounds impossible

Reply 2

Robusty

littleone

heya guys am stuck on the 1st and last questions from the electrochemistry booklets....please help as i have no idea where to start tese...i attach a copy of the questions!
thank u!

ah, well thats not too bad:

you need to know how many electrons are involved in the question (both question 1 and 3 are similar)

by loking at the equation, you can see that TWO electrons are involved since I2 -> 2I- so an iodine molecule plus two electrons equal two iodide ions.

One you know the number of electrons involved, then you'll know how many faradays are used and just refer back to the example in the notes and you'd be fine. :wink:

hope this helps :smile:

p.s. what group are you in anyway??

Reply 3

rupalini

2

You are given a 1 mL vial containing a concentrated solution of acetylcholine chloride (ACh: Mol. Wt.=181.7). After diluting this solution 1000 fold, you add 0.1 mL of it to a 25 mL organ bath, to give a final bath concentration of ACh of 10 nM.

Calculate for
the original stock ACh solution (a) the millimolar concentration (b) the concentration expressed in milligrams/mL. Show all your calculations.

Can anyone help with the above question? When i tried it I got a very weird answer.. it is from the june 2004 paper 1 and thats the only paper where the answers arent on blackboard!! Please help! Thanks

Reply 4

londonguy

Use the C1V1 = C2V2 fomula

i.e

25 * (10nm/1000) = 0.1 * C2

Answer = 2.5MicroMolar. Remember it has been diluted 1000 times, therefore multiply it by a 1000 (I think - not sure..someone needs to confirm this), and u get 2.5 milimolar, and that is the concentration of the original ACh

Miligrams/ml.....hmm... since the concentration is 2.5 milimolar in 1 ml, using the formula c= n/v, I work out the number of moles to be 2.5 milimolar, therefore, multiply that by 181.7 which is 0.45425grams which is 454.25 miligrams.

I cant promise this is the correct answer. Someone like robusty will help us!

Reply 5

mysteryman

2

rupalini

You are given a 1 mL vial containing a concentrated solution of acetylcholine chloride (ACh: Mol. Wt.=181.7). After diluting this solution 1000 fold, you add 0.1 mL of it to a 25 mL organ bath, to give a final bath concentration of ACh of 10 nM.

Calculate for
the original stock ACh solution (a) the millimolar concentration (b) the concentration expressed in milligrams/mL. Show all your calculations.

Can anyone help with the above question? When i tried it I got a very weird answer.. it is from the june 2004 paper 1 and thats the only paper where the answers arent on blackboard!! Please help! Thanks

You basically have to work the question backwards from the 10nm bath concentration. I find it much easier to use dilution factors than the "much dreaded C1V1 equation" lol.

I get 2.5mM as concentration 454mg = 450mg as the answer. So basically as londonguy said.

Reply 6

rupalini

2

mysteryman

You basically have to work the question backwards from the 10nm bath concentration. I find it much easier to use dilution factors than the "much dreaded C1V1 equation" lol.

I get 2.5mM as concentration 454mg = 450mg as the answer. So basically as londonguy said.

i emailed dr pearce and he confirmed my answers were correct..

a) 2.5mM
b) 0.45mg/ml-1

Reply 7

londonguy

whoops!

450mg - I must have forgotten to express it as a concentration rather than how much mg it is!

So lets get it straight...

if 450mg is how much there is in 1ml, therefore, one has to express it as mg/ml.

Im trying to work out where did 0.45mg/ml-1 come from.

Reply 8

mysteryman

2

londonguy

whoops!

450mg - I must have forgotten to express it as a concentration rather than how much mg it is!

So lets get it straight...

if 450mg is how much there is in 1ml, therefore, one has to express it as mg/ml.

Im trying to work out where did 0.45mg/ml-1 come from.

450mg is how much there is in a litre i.e. in ml you need to divide by 1000 to get 0.450mg/ml

Reply 9

Robusty

thats right, got the same answer:

a) use the C1V1 = C2V2 fomula:
C1 = X
V1 = 0.00001 dm3
C2 = 10x10-9 M
V2 = 0.025 dm3

work out C1 first, which is 2.5x10-6 M, x1000 to get original conc.

then:

b) 181.7 x 2.5x10-3 to get 0.45425 g/dm3, which is basically the same as 0.45425 mg/ml. Since they only have 2 s.f in the question, therefore we need to have the same s.f. in our answer, i.e. 0.45mg/ml

Reply 10

londonguy

hey...how was that paper 1 today?

I thought it was ok...I was lucky because I didnt do revision on antibodies and a few other stuff and they didnt come up! :-)