Experiment 1: Purification of Benzoic Acid by Recrystallization
Purification - physical separation of impurities and by-products from an impure sample pure substance.
*Distillation- for liquid compounds,
*Recrystallization- solid compounds
- precipitation of a solid cpd. from a saturated solution in the form of crystals through cooling or evaporation -7 steps: 1) selecting the appropriate solvent
2) dissolving the solute
3) decolorizing the solution
4) removing suspended solids
5) crystallizing the solute
6) collecting and washing the crystals
7) drying the product
- 2 Kinds of Impurities: 1) more soluble
2) less soluble than the main component
- impure solid first dissolved in an appropriate solvent when heated - appropriate/ideal solvent = dissolves component to be purified when hot (boiling point), = does NOT dissolve component to be purified when cold= room temp. or 0oC = either dissolves impurities when cold OR does NOT dissolve impurities when hot - components will then crystallize in its purer form as the solution is cooled. Less soluble component will crystallize first
As the crystals are formed, the correct molecules that will fit in the crystal lattice are selected while the wrong molecules are ignored, resulting in a pure solid - Depends on: 1) solubility of the compound
2) differences in solubility of the desired solutes & impurities dissolved in the solvent - Common Solvents Used:
| Boiling Pt. (oC)
| Very Polar
| Slightly Polar
| Slightly Polar
0.100 g of powdered impure benzoic acid, 1 small piece of boiling chip, and 2 mL of distilled water were added respectively into a vial. While swirling the mixture constantly, it was then heated to boiling until the benzoic acid had dissolved completely in the water.
The solution was then cooled by adding a few drops of distilled water. A pinch of activated carbon was added after. Then, the mixture was heated to boiling for a second time for a few minutes until the volume of the mixture had decreased by approximately 1 mL.
The hot mixture was then immediately poured into a 5mL disposable syringe (without the needle) with a cotton plug inside at the bottom of the syringe tube to act as a filter. The piston of the syringe was then pressed to force the liquid through the filter, collecting the filtrate into another vial.
The hot saturated solution or filtrate was then cooled to room temperature before sealing the vial and slow-cooling it in an ice bath. The crystals yielded were collected in a pre-weighed filter paper on a 50 mL beaker (as an improvised filter paper support). Small increments of ice-cold water were added to rinse the vial to get the remaining crystals.
The filter paper containing the collected crystals was then carefully taken out from the filter system, then was folded with the crystals inside, and then pressed in between folded tissue paper to squeeze out excess water. After allowing the filter paper and crystals to dry up completely, the filter paper with the crystals was weighed. The weight of the filter paper alone was subtracted from the obtained weight to get the weight of the yielded crystals. Percent recovery was computed for by dividing the weight of the yielded crystals by the weight of the impure benzoic acid.
From the 0.100 g of the impure sample of benzoic acid, 0.030 g of pure benzoic crystals was yielded, having a percent recovery of 30%.
The chemical equation for the reaction of benzoic acid with water is as follows: C6H5COOH(aq) + H2O(l) C6H5COO-(aq) + H3O+(aq).
It is a reversible reaction allows the dissolved benzoic acid to...