Question

Isolation of Caffiene from tea leaves

What percent recovery could be expected if 0.188 g of caffeine was initially dissolved in 120 mL of water and then extracted with a single 60 mL portion of methylene chloride? (The partition coeffiecent between caffeine and methylene chloride is 8)

What percent recovery could be expected if 0.188 g of caffeine was initially dissolved in 120 mL of water and then extracted with three, 20 mL portions of methylene chloride?

(The partition coeffiecent between caffeine and methylene chloride is 8)

6. Would the apparent percent recovery be higher, lower, or unchanged if a student did the following (answers must provide an explanation founded in the underlying chemistry):

a. Used room temperature water to brew the tea?

b. Did not add calcium chloride after washing the dichloromethane with two 20 mL portions of 6M NaOH?

Answer 1

Formula:

Fraction remaining in water = (V1)/(V1 + KV2)

V1 is volume of water, V2 is volume of methylene chloride, and K is partition coefficient.

Fraction remaining in water = (120 mL)/(120 mL + 8*60 mL)

                                            = 0.2

Fraction of recovery = 1-0.2 = 0.8

Recovery = 0.8*0.188 g = 0.1504 g

%recovery = (0.1504 g)*100/0.188 g = 80%