Chemistry 245 Lab
Author: Kylie Bui
Team Members: Nhien Tran and Veronica Barrezueta
Introduction: the purpose of this lab was to purify an impure solid through the process of recrystallization through the help of hot gravity filtration and vacuum filtration. Recrystallization is done by dissolving solids to remove its impurities based on their differences in solubility. In this experiment, two main objectives include comparing the purities between the desired benzoic product versus its impure and calibrated samples and also determining the product recovery percentage. The expected outcome is that while the final recrystallized product will be purer than the impure/crude sample, its purity still cannot compare to the calibrated sample’s purity and the recovery rate should be around 30-40%.
1. Chemical(s): benzoic acid
2. Materials: Mel-Temp Thermo Model No: 1001D, Serial No: 1877080835168, assembled in Malaysia. Sold at 2555 Kerper Blvd, Dubuque IA, 52001. http://www.labx.com/
Thermometer, hot plate, ring support, stemless funnel, Erlenmeyer flask, water, stirring rod, filter paper, envelop.
Weigh about 1.0 grams of benzoic acid and record exact mass. Next, on the heating hot plate, dissolve the sample in 20 mL of near boiling water in the flask. Use a stirring rod to crush all chunks of benzoic powder and swirl the solution. Add enough hot water and swirl solution on hot plate until all is dissolved.
Filter the hot solution with the gravity filtration setup to remove impurities. If white crystals remain on paper, add a bit more hot water to re-dissolve. **Do not add too much water and keep setup warm throughout the filtration process.
Then, continue to dry the crystals by vacuum filtration. Clamp a 125 mL Erlenmeyer flask to a ring stand, then connect a thick vacuum hose to the side arm and the other end to the vacuum outlet. ***Note that a thin hose will collapse. Next, place a rubber adapter and then a stemless funnel on top of the flask. Then, place a wet piece of fluted filter paper to cover up the holes in the funnel. Begin the vacuum filtration process and rinse the crystals with about 5 mL cold water. Leave them to dry in an envelope until next class to obtain the melting point. ***Do not forget the rubber adaptor.
Results and Discussion:
Experimental and calibrated melting point ranges for pure, impure, and recrystallized benzoic acid:
Observed melting point
Calibrated melting point
Benzoic acid (data from week 1)
119 ⁰C- 122.9⁰C
119.5°C – 123.4°C
Crude benzoic acid
109.5°C – 115.8°C
110.0°C – 116.3°C
Recrystallized benzoic acid
116.0°C - 119.8°C
116.1°C – 119.9 °C
The outcomes of this experiment fulfilled expectations, which consisted of purifying an impure organic compound, benzoic acid, comparing its purity level to the initial and crude samples, and also determining the % recovery. We started out the experiment by dissolving a small sample of impure benzoic acid. Due to the different solubility between pure and impure chemical substances, the impurities were able to separate themselves from the pure materials when the solution dissolved. Next, we begun the process of hot gravity filtration to further separate the impurities (supposedly still soluble) from the desired purified benzoic acid as it slowly recrystallized when the solution cooled down. At this point, we added about 2-5 mL of hot water to re-dissolve the solution and keep the filtration process going as planned, which perhaps was not