Question

One gram of caffeine dissolves in 55 mL of water, 5.5 mL of chloroform, 530 mL of diethyl ether, and 100 mL of benzene. Calculate the solubility of caffeine in these four solvents in mg/mL. Estimate the partition coefficient of caffeine between chloroform and water, diethyl ether and water, and benzene and water.

Which solvent would you choose to extract caffeine from an aqueous solution? why?

Answer 1

Solubility of caffeine in water:

mass of caffeine = 1 g = 1000 mg

volume of water = 55 ml

solubility of caffeine in water = 1000 mg/55 ml = 18.18 mg/ml

Solubility of caffeine in chloroform:

mass of caffeine = 1 g = 1000 mg

volume of chloroform= 5.5 ml

solubility of caffeine in chloroform = 1000 mg/5.5 ml = 181.82 mg/ml

Solubility of caffeine in diethyl ether:

mass of caffeine = 1 g = 1000 mg

volume of diethyl ether= 530 ml

solubility of caffeine in diethyl ether= 1000 mg/530 ml = 1.89 mg/ml

Solubility of caffeine in benzene:

mass of caffeine = 1 g = 1000 mg

volume of benzene= 100 ml

solubility of caffeine in benzene = 1000 mg/100 ml = 10.00 mg/ml

Partition coefficient of caffeine between chloroform and water = solubility of caffeine in chloroform/solubility of caffeine in water

= 181.82/18.18 = 10.00

Partition coefficient of caffeine between diethyl ether and water = solubility of caffeine in diethyl ether/solubility of caffeine in water

= 1.89/18.18 = 0.10

Partition coefficient of caffeine between benzene and water = solubility of caffeine in benzene/solubility of caffeine in water

= 10.00/18.18 = 0.55

If the partition coefficient is more than 1, then it prefers organic phase.

If the partition coefficient is less than 1, then it prefers aqueous phase.

To separate caffeine from an aqueous solution, chloroform is the best solvent because Partition coefficient of caffeine between chloroform and water is more than 1.