college orgo
posted by sara .
calculate how to make 10 mL of 15:85 ethyl acetate:hexane solution
from 2 mL of 5:95 ethyl acetate:hexane, using pure hexane and pure ethyl
acetate.

I'm sure this is not the shortest way to work this problem but I think this will work.
5+95 is 1 part EtAc + 19 parts hexane. Therefore, 20 parts is 2 mL.
2/20 = 1/10 = 0.1; i.e., it is 0.1 part EtAc + 1.9 parts hexane (total of 2 mL).
If we are to increase the concn (5% to 15%) the only way we can do that is to add pure EtAc; therefore I let x = amount EtAc we need to add to obtain 15%.
[(0.1+x)/(2+x)] = 0.15
Solve for x. I get 0.235 part (0.235 mL) EtAc. [Note: To check this out we would have [(0.235+0.1)/(2.00+0.235)] = 0.15 so we're on the right track. Now we want to scale it up how much? By a factor of 10 mL we want/2.235 we have which is a factor of 4.47.
4.47*0.335 = 1.497 mL EtAc and
4.47*2.235 = 9.99 EtAc + hexane.
Check it to make sure it is 15%.
1.497/9.99 = 0.15