Hello,
Im new to this forum.

In an extraction of caffeine from water, 50ml of ac aqueous solution was extracted with two  25 ml portions of chloroform. from the solubilities of caffeine the distribution coefficient of caffeine, K was calculated to be CHCL3/H2O = 18/2.2 Assume that the 50 ml of aqueous solution contained 1 g of caffeine. Find the total eight of caffeine transferred to the chloroform in the two extractions.

I have no clue how to even start this problem.

you can use this formula
w1 (first extraction) = w0 ( Kv/Kv + s)^1
here w0 = 1g (weight of solute in aq layer)
v is the volume of aq layer = 50mL
s is the vol of organic solvent = 25 mL
K is given = 18/2.2
so now you can try to take out weight for first extraction

So I should be able to the something for the second extraction?
But then do I have to use the same volume? Since 25 ml was extracted once?

Thanks for the quick reply

for the second extraction
volume of the aq layer  and orgo layer  remain the the same
only weight of the solute in aq  layer is changed
subtract mass which is extracted in first extraction from the total mass
1-w1 = w0 now
use same equation to do calculations 

using the formula you gave me
w1 = w0 ( Kv/(Kv + s))^1

first extraction gives me 0.94g which some how does not make sense to me , i might be doing something wrong here.

am i doing the right thing adding the denominator Kv with s?

because of i dont add those two then it comes to 26g

I think you are doing some error in doing calculations

total weight of two extractions comes to 0.99g,
I dont know what im doing wrong here.

for the first extraction I am slo getting 0.9424g
now weight of Caffeine left in aq layer = 1- 0.9424 = 0.05759g
now change your  w0 = 0.05759 g
second extraction gives you 0.05427g
total mass extracted = 0.9424g  +  0.05427g

yeah thats what i got as well

can i get help with this as well?
at the molecular level, why lowering the pressure lower the boiling point of a liquid ( this one telates to the use of the rotary evaporator)

boiling point is the temperature at which the vapor pressure of the liquid becomes equal to atmospheric pressure
Hence  if pressure is lowered ,you need less temp to reach the vapor pressure which is equal to external pressure

Thanks that helped alot.

Try it with tea:

Take 12 teabags boil in water (1.5 cup), add some gypsum to counter the tannin for later extraction of caffeine.

Remove the teabags, let it cool, and use chloroform to extract the caffeine. Three extractions of 25ml should work.

Evaporate on a water bath, and recrystallize if preferred.