Question

0.070g of caffeine is dissolved in 4.0ml of water. Calculate the total amount of caffeine that can be extracted by:

a) a 6.0ml portion of methylene chloride

b) three 2.0ml portions of methylene chloride

Caffeine has a distribution coefficient of 4.6, between methylene chloride and water.

Answer 1

distribution coefficient = 4.6 = gm of caffeine dissolved in ml of methylene chloride/ gm of caffeine dissolved in ml of water

let x= gm of caffeine dissolved in methylene chloirde, 0.07-x= gm of caffeine dissolved in water

when 6m of methylene chloride is used, 4.6= x/6/(0.07-x)/4

4.6= (4/6)*x/(0.07-x)

4.6*1.5= x/(0.07-x)

6.9= x/(0.07-x)

6.9*(0.07-x)=x

6.9*0.07= 7.9x

x= 6.9*0.07/7.9 =0.061 gm, caffeine extracted =0.061

2. when 2 ml portions od methylene chloride is used, x= amount of caffeine in the methylene chloride

4.6 = (x/2)/(0.07-x)/4

2.3= x/(0.07-x)

2.3*(0.07-x)= x

x= 2.3*0.07/3.3 = 0.049, hence caffeine to be extracted is that remains in the aqueous phase= 0.07-0.049 =0.021

this is treated with 2 ml of methylene chloride again

hence if x= gm of methlylene chloride extracted

4.6= (x/2)/(0.021-x)

2.3= x/(0.021-x)

2.3*0.021-2.3x= x

x= 2.3*0.021/3.3 =0.0161, caffeine to be extracted further= 0.021-0.0161= 0.0049

if x = amount of caffeine extracted in the 3rd extraction

4.6= (x/2)/(0.0049-x)/4

2.3= x/(0.0049-x)

2.3*0.0049= 3.3x

x= 2.3*0.049/3= 0.0037, remaining methylene chloride= 0.0048-0.0037=0.0011

caffeine extracted =total of caffeine- caffeine remained unextracted =0.07-0.0011= 0.0689 gm