Question

1. Assume that the solubility of caffeine in water is 4.0 g/100 mL and in methylene chloride it is 16.0 g/100 mL at 25°C. What is KD for this system? 3. Once you calculate KD, calculate how much caffeine would be extracted by mixing 1.0 g of caffeine in 100 mL of water and performing three consecutive extractions with 25 mL CH2Cl2 each time. Show your work.

2. Assume you changed the solvent to ethyl acetate which has a KD for caffeine of 10. How much caffeine would you extract after three extractions 25 mL each? Show your work.

3. Is caffeine an acidic or basic compound? Answer using a balanced equilibrium equation for caffeine in water that includes the correct structure for caffeine.

Answer 1

Q1.

Sw = 4 g/ 100 mL in water

So = 16 g/ 100 mL in methylene Chloride

Find Kd:

Kd = solubility in organic solvent / solubility in aqueous solvent = So/Sw

Kd = (16)/4) = 4

Kd = 4

ii)

how much caffein if we mix 1 g of caffein in 100 mL of water, then 3 extractions with V = 25 mL each...

so

Concentration of C in water = 1/100 = 0.01 g of caffine / mL water

C of caffeine = 0 /25 mL

after extraction:

Cw = (1-x) / 100

Co = x / 25

Kd = Cw/Co

4 = (x / 25) / ((1-x) / 100)

4/100 * (1-x) = x/25

0.04 - 0.04x = 0.04x

0.08x = 0.04

x = 0.04/0.08 = 0.5

so 25 mL of caffein will extract 0.5 g

then...

Cwater after 1st extraction --> 1-0.5 = 0.5

repeat:

4 = (y / 25) / ((0.5-y) / 100)

4/100 * (0.5-y ) = 0.04*y

0.04 (0.5-y ) = 0.04*y

0.5-y = y

2y = 0.5

y = 0.5/2 = 0.25 g extracted

Total mass = 0.5 + 0.25 = 0.75 g extracted

finally:

4 = (z / 25) / ((0.25-z) / 100)

0.04*(0.25-z) = 0.04z

0.01 - 0.04z = 0.04z

0.08z = 0.01

z = 0.01/0.08 = 0.125 g

so...

Total = x+y+z = 0.5 + 0.25 + 0.125 = 0.875 g of caffeinwill be extracted

Q2.

Repeat with Kd = 10

Kd = Cw/Co

Kd= (x / 25) / ((1-x) / 100)

Generic:

Kd * (Co-x ) / (100) = x/25

Kd * (Co-x ) = 4x

Kd*Co - Kdx = 4x

x(4+Kd) = Kd*Co

x = Kd*Co/(4+Kd)

if Kd = 10

x = 10*Co/(4+10) = Co * 0.7142 g if Co = 1 g --> 0.7142 g

repeat

y =  Co * 0.7142 = (1-0.7142) *0.7142 = 0.204118 g

finally

z = Co * 0.7142 = (1-0.7142-0.204118 ) *0.7142 = 0.05833g

total mass = x+y+z = 0.7142 +0.204118 +0.05833 = 0.976648 g will be extracted

3

caffeine is basic:

C8H10N4O2 + H2O <------> C8H11N4O2+ + OH-

it increases OH- in solution so it is ph> 7, basic