Lab Report 2- Recrystalization PDF

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This is the second lab report for the recrystallization experiment ...

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CH 203 2.13.2020

Recrystallization Introduction and Theory (1 point) One of the techniques that can be used to purify solid compounds is recrystallization. Using this technique, the impurities are dissolved out of the sample. This process is based on the principles of solubility because some compounds are more soluble in hot solvents than they are in cooled solvents. There are three possible common situations where recrystallization can be used for purification process. In the first case, the impurities and substrate are soluble in hot solvent, but only the impurities are soluble in cooled solvent. In the seconds case, the impurities are soluble in the hot solvent. In the third case, the substrate and impurities are soluble in hot solvent, but the substrate is soluble in cooled solvent. For this experiment, the purification started with dissolving the solute mixture in a hot solvent, so the first impurity could be separated from the solution by filtration. I – Objective (1 point) The purpose of this lab was to purify an impure sample of acetanilide by recrystallization technique. For evaluating the purity of the sample, the melting point was determined at the end of the experiment. Then the solution in the filter flask for the second time was heated again until the solid dissolved. After waiting for the solution to reaches to room temperature, it was placed in an ice bath so that the particles of the compound began to come out of solution forming crystals. Then by the second filtration, the crystals were separated from the solution. Then by identifying the melting point of the crystal, the purity of the sample could be determined by comparing the melting point range of it with the melting range point of the acetanilide from the table.

Mass of the starting sample: 1.203 g Mass of purified crystals: 2.580 g Melting point of the purified crystals: 112.8-117.5 °C Percent recovery of acetanilide from the sample: 54.3 %

II – Changes in Procedure (0.5 point) None

III – Observations and Results (1 point) The impure sample of acetanilide was white powder. By placing the compound in the hot solution for the fist time, the impurity was removed from the solution by filtration because the impurity, which was sand, is insoluble in hot solvent. After the first filtration, the solution was white and it were reheated. Then the solution was cooled down first by room temperature and second by ice bath. The crystals appeared in the flask that was in ice bath, and they were clear and small.

IV – Discussion (3 points) For purifying the impure sample of acetanilide, which was contaminated with sugar and sand, was placed in a hot solution. The sand was separated from the solution by filtration. After that, the solution was heated again and cooled down in order to have crystals and be recovered by filtration. The melting point range confirmed that the recrystallization was done accurate enough. The range that was observed during the experiment was from 112.8-117.5 °C and the known

melting point for acetanilide is 114.3 °C. The percent recovery of acetanilide that was calculated !.#

%$by dividing the ass of the crystal by the total mass if the sample times 100 was 54.3% ( &.'!( 𝑥100 = 54.3%). The results from this experiment was conclusive.

V – Questions (1 point) Post Lab Questions

1. Assuming that you recovered 100% of the acetanilide in the sample, how much of the mass of the original sample was made up of impurities?

If the sample contains 80% acetanilide and 20% sugar and sand -> 1.203*0.8=0.9624 g 1.203-0.9624=0.2406 g -> is the mass of impurities.

2. Based on the melting point obtained for your purified sample, what can you conclude about the purity of the sample?

Because the melting point range that obtained from purified sample was narrow (no more than 5°C difference) and it was close to the known melting point, so I can conclude that the sample is pure.

3. Why was it necessary to filter very quickly to remove the sand? If the solution cools down, then the sand would no longer separate from the compound. So the crystals that will be obtained will have sand.

VI – Conclusion (1 point) In this experiment, recrystallization was used as on of the techniques for purification of the impure sample of acetanilide. The percent recovery for the experiment was 54.3%. For verifying the purity of sample using recrystallization technique, the melting point of the purified should be measured and compared to the known melting point. This range was from 112.8-117.5 °C which verifies the purity.

VII – References (0.5 point) https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Mod ules_(Physical_and_Theoretical_Chemistry)/Physical_Properties_of_Matter/Solutions_and_Mixtures/Case_Studies/ RECRYSTALLIZATION

http://www.utdallas.edu/~scortes/ochem/OChem_Lab1/recit_notes/recit_exp3_cryst.pdf\\ http://www.pitt.edu/~ceder/add_info/recrystallization.html...