Question

After a synthesis, a student obtains 1.0 g of crude and impure product. In an attempt to purify the crude material, the student dissolved the aforementioned 3.0 mg into the minimum amount of recrystallization solvent necessary at 25°C.   Next, the student cooled the flash containing the mixture in an ice-water bath for 15min while scraping the inside of the flask with a glass stir rod until no more crystal formation was observed. The solution was filtered, and the crystals were rinsed with ice-cold EtOH. After drying the product, the student obtained 0.14 g of pure product. Upon comparing results with the remainder of the class, this student found that their purity was on par with the rest of the class, but the percent yield was substantially lower. Why did the student obtain such a low percent yield?

Answer 1

Recrystallization steps include

1) Dissolving solute in minimum amount of solvent near it's boiling point

2) cooling the solution to achieve supersaturation

3) crystal growth and filtering of impurities

4) drying

The student has same purity of compound compared to others but percent yield has decreased significantly

The probable reasons are many, mainly two are given below

1) The student may have dissolved solute below the boiling point of solvent , therefore solvent is not near it's boiling point.

The solvent solubility increases with temperature and same amount of solvent dissolves more solute at higher temperature than at lower temperature. Using solvent well below it's boling point decreases percent yield resulting in dissolution of less amount of solute.

2) The student uses glass stir rod inside the flask to induce crystallization. Significant number of crystal formation occurs on the glass rod which is well immersed in the flask. This amounts to solute losses which deceeses percent yield.

Other reasons are solvent spillage and solute losses

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