E1 Purification of Benzoic Acid

Which statement is FALSE about recrystallization as a purification technique?

• Exploits difference in solubility of solute in a given solvent at varying temperatures

• Exploits difference in solubility of desired compound and contaminants in a given solvent

• Exploits the high vapor pressure of a solid at a temperature below its melting point

• Exploits the ability of a compound to form pure crystals upon cooling from a saturated solution

Which is NOT a criteria in choosing an appropriate recrystallization solvent?

• Desired solute is sparingly soluble at room temperature but soluble at high temperature.

• Solvent does not react with the solute.

• Solvent is moderately volatile and non-flammable.

• Boiling point of solvent is higher than the melting point of the desired solute.

Which of the following statements is NOT true when performing recrystallization?

• Adding a small amount of activated charcoal adsorbs colored impurities.

• Using a 3:1 filter paper maximizes surface area for faster collection of the filtrate.

• Pre-heating the receiving flask prevents premature recrystallization during hot filtration.

• Allowing the hot solution to cool slowly promotes crystal growth.

If no recrystallization is observed, which of the following can be done?

• If the solution is unsaturated, add a small crystal of pure substance.

• If the solution is unsaturated, scratch the walls or bottom of the flask.

• If the solution is unsaturated, boil off the excess solvent.

• If the solution is unsaturated, mix the solution thoroughly.

Which of the following is TRUE about recrystallization and sublimation?

• Recrystallization usually offers higher purity than sublimation.

• Sublimation is applicable for a wider range of compounds than recrystallization.

• Recrystallization does not require solvent unlike sublimation.

• Sublimation usually offers higher purity than recrystallization.

Which of the following will INCREASE the apparent %recovery of recrystallized benzoic acid?

• Not adding activated charcoal

• Using a fluted filter paper for cold filtration

• Not pre-heating the receiving flask for hot filtration

• Using a long-stemmed funnel for hot filtration

Which is the BEST recrystallization solvent for solute A?

• W

• X

• Y

• Z

If 100 mL of solvent W was used to recrystallize 1.0 g solute A, what will be the %recovery of A?

• 40%

• 55%

• 60%

• 90%

If 100 mL of solvent Y was used to recrystallize 1.0 g solute A, what will be the %recovery of A?

• 20%

• 80%

• 40%

• 100%

Caffeine has a theoretical melting point of 235°C. Given the following experimental melting ranges, which sample contains the HIGHEST percentage of caffeine?

• Sample A: 229-231°C

• Sample B: 232-234°C

• Sample C: 226-234°C

• Sample D: 224-226°C