CALCULATING PARTITION COEFFICIENT - Organic Chemistry Lab

Question

1. Calculate the partition coefficient of caffeine in water/methylene chloride from the following data: 10.3 g/100 mL for methylene chloride and 2.04 g/100 mL for water.

 

MY ANSWER: 

 

K = (10.3g/100mL) / (2.04g/100mL) 

K = 5.049019608 

K = 5.05

2. In a very controlled experiment (not ours), a student is sure that 0.500 grams of caffeine is dissolved in 125 mL of water. The student extracts the caffeine using a single extraction of 21 mL of methylene chloride. How much caffeine is in the methylene chloride? How much remains in the water? (use the partition coefficient calculated in #1)

 

I'm not sure how to set this problem up... Any help would be greatly appreciated. 

Answer 1

This set up involved a little bit of algebra.  We know some of the sample with stay in the water and some will go into the CH₂Cl₂ layer.  Based upon your value for K, it is more soluble in CH₂Cl₂ so we would expect a higher amount in that layer at the end of our calculation.

 

Let's define x as the mass in the H₂O layer.  And then 0.500-x will be the mass in the CH₂Cl₂ layer 

 K = concentration in CH₂Cl₂      = 5.05  =      (0.5-x)/20mL          =  (0.5-x)(125mL)   =  5.05

       concentration in H₂O                               x/125 mL                        20x   

 

   Solve for x = 0.28 g in H₂O    And 0.5g -0.28 g = g in CH₂Cl₂