recrystallization of Benzoic acid

Topics: Solvent, Solubility, Solution Pages: 6 (879 words) Published: November 2, 2014
Mabel M. Kimble

September 21, 2014

Chemistry 234

Recrystallization of Benzoic Acid


The recrystallization of Benzoic Acid was performed to fully master the ability to purify a solid compound. This form of purification technique is especially important in chemical identification, where the melting point of the compound is taken to confirm its purity. In order to perform recrystallization a solubility test much be completed. Four solvents (95% Ethanol, Petroleum Ether, Acetone and distilled water) were used to identify which one is an ideal solvent. An ideal solvent is insoluble in room temperature and soluble in hot solvent. 0.5mL of each solvent were added into four reaction tube with 25mg`of pure Benzoic acid. After careful observation, distilled water fit the requirement. During recrystallization, 0.5 grams of Benzoic acid were added into a 50mL flask with 10 mL distilled water. After performing this experiment, the first crop had a mass of .334 grams with a second crop of 0.0grams. The percent recovery yields 66.8%, which means that according to expected value of 70% this data confirms that the Benzoic is pure. Finally, the melting range was between 120 to 122 degrees which again confirms that the recrystallized Benzoic acid has been purified completely.


Recrystallization is a commonly used method of purifying solid compounds. Insoluble impurities get extracted from the compound by dissolving the mixture in hot solvent. The soluble impurities are dissolved in a high temperature environment and the solvent is cooled in room temperature while Benzoic acid crystallizes. The crystalized Benzoic acid is then placed in an ice bath to cool it further. The crystals are then filtered out using the vacuum filtration provided in lab. The filter paper is taken out and crystallized Benzoic Acid is dried out. Overall, the prediction for this experiment would be that the percent recovery will yield a low outcome.

Purifying Benzoic acid by using heat through hot solvent, filtering the crystal, cooling and drying could have resulted in some of the liquid staying in the gaseous state (evaporation).


Solubility test

In order to find the ideal solvent for recrystallization 25 mg of pure Benzoic acid with 0.5mL of each solvent (95% Ethanol, Petroleum Ether, Acetone and distilled water) is added into a reaction tube. Four tubes is placed in hot solvent and room temperature. The goal is to identify an ideal solvent in which Benzoic acid will be insoluble in room temperature and soluble in hot solvent. In room temperature, distilled water and petroleum ether were insoluble while, acetone and 95% ethanol were soluble. In hot solvent, acetone and ethanol were insoluble while, petroleum ether and distilled water were soluble. Both tubes were placed in an ice bath for crystallization and both produced the same results however, distilled water is more green and therefore, is the ideal solvent for this experiment.


In recrystallization, 0.5 grams of impure Benzoic acid with 10mL distilled water was added into a 50mL flask using a plastic pipette. 10 Norit of activated carbon was then added to absorb color impurities of the reaction. The flask was placed on top of sand bath to boil. Once the solution has completely dissolved, gravity filtration was used to filter the crystals away from the activated carbon. The solution was placed in sand bath again to dissolve. Once the flask shows a clear solution it is placed on the counter top for cooling and crystallization. To further cool the flask, the solution is placed on an ice bath. A new flask with Hirsch funnel and filter paper was then placed in a clamp while, crystalized pure Benzoic acid was filtered out with using vacuum filtration and dried out. The mother liquor was again placed in the sand bath to boil until it reaches 3mL. A second crop was then collected by using vacuum filtration again and collecting...
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