Question

A student purified a 500-mg sample of phthalic acid by recrystallization from water. The published solubility of phthalic acid in 100mL of water is 0.54 g at 14C and 18 g at 99C. The smallest volume of boiling water the student could use to dissolve 500 mg of phthalic acid is 2.778 mL of water.

Dissolution of phthalic acid in boiling water produced a dark-colored solution. The student allowed the solution to cool, added several spatulas full of activated carbon, and heated the mixture to boiling. After gravity filtration, the clear and colorless solution was allowed to cool to room temperature. Crystals formed, and the student isolated 380 mg of phthalic acid.

a) Calculate the percent recovery of phthalic acid in this experiment.
b) Suggest one or more procedural errors the student made that could be responsible for some loss of phthalic acid.

Answer 1

Given:
the published solubilities of phthalic aicd are
0.54g/100mL at 14C
18g / 100mL at 99C

a) Since the solubility of the acid is more when water is at 99C therefore
smallest volume of boiling water = 500mg / (18g/100mL)
= 0.5g / (18g /100mL)
= 2.78mL

Percent recovery of the acid = {(380mg) / (500mg)}x100
= 76%

b) possible causes is that recrystallization...and during filteration.