Question

A solid substance D is soluble in ethanol to the extent of 2.6 mg / mL of ethanol at 25 C and 12.0 mg / mL of ethanol at 78 C. You have been provided with a sample that contains 40.0 mg of D and 5.0 mg of an impurity E. By showing all the calculation steps, describe how you would be able to purify D if E has the same solubility behavior as D in ethanol. Indicate the number of crystallization steps required to purify D, as well as the amount and percent recovery of pure D obtained after the crystallization steps have been completed.Show all calculations.

Answer 1

Both the substance D and E has the same solubility behaviour.

Total amount of substance (D+E)= 40 mg + 5 mg= 45 mg

As we know that 12.0 mg is soluble in 1 ml of ethanol at 78 ⁰C. In order to dissolve 45 mg of total substance requires 3.75 ml of ethanol at 78 ⁰C. Upon cooling to 25 ^oC, 35.25 mg of the compound will precipitate and 9.75 mg stay in solution. (after first recrystallization)

Among 9.75 mg stay in solution, 5mg of impurity substance E and 4.75 mg of substance D are present in 3.75 ml of ethanol.

percent recovery of pure compound D= (amount of D precipitated/ total amount of substance) x 100

percent recovery of pure compound D= 78.33 %