Question

0.20 g of caffeine dissolves in 10 mL of room-temperature water, while 0.66 g of caffeine will dissolve in 10 mL of boiling water.

A. If a 5.0 g sample of caffeine is to be recrystallized from water, calculate the minimum amount of water required.

B. A student uses 75.0 mL of water in order to recrystallize a 5.0 g sample of caffeine. Once cooled to room temperature, calculate the amount of caffeine remaining in the water and thus lost in the recrystallization? How much of caffeine can the student expect to recover?

Answer 1

Solution :-

At room temperature 0.20 g caffeine is soluble in 10 ml water

And 0.66 g caffeine is soluble in 10 ml boiling water

A)If a 5.0 g sample of caffeine is to be recrystallized from water, calculate the minimum amount of water required.

Solution :- amount of water needed to recrystallize the 5.0 g caffeine is

0.2 g is soluble in 10 ml water and 0.66 g soluble in boiling water

Therefore amount of the caffeine recrystalized from the 10 ml water is 0.66 g – 0.20 g = 0.46 g

Therefore 0.46 g = 10 ml

                   5 g = ? ml

                 = 5 g * 10 ml / 0.46 g

                = 109 ml water

B. A student uses 75.0 mL of water in order to recrystallize a 5.0 g sample of caffeine. Once cooled to room temperature, calculate the amount of caffeine remaining in the water and thus lost in the recrystallization? How much of caffeine can the student expect to recover?

Solution :-

10 ml = 0.2 g

Therefore

75.0 ml = ? g

75.0 ml * 0.2 g / 10 ml = 1.5 g

Therefore 1.5 g caffeine will remain in the water.