Explain the following answers regarding the crystallization procedures that are

Question

Explain the following answers regarding the crystallization procedures that are being performed by a student. (DO NOT EXCEED THE SPACE GIVEN FOR THE ANSWERS). a) While heating the impure solid with the solvent a significant amount of the solid remained undissolved in the solvent. The student attempted to perform a hot filtration using gravity filtration. While performing the hot filtration a large amount of the solid was formed on the filter paper. The hot filtrate was then immediately cooled by placing the container in an ice-bath. The resultant solid was then collected by means of gravity filtration. Give all the reasons as to why the percent recovery of the purified solid obtained by the student was so low. (You may answer the question in a point-wise manner). b) After the impure solid was completely dissolved in the hot solvent, the student immediately placed the flask containing the hot solution into an ice bath. Within a few seconds a large amount of the solid formed, which was then collected by means of vacuum filtration. The student observed that the percent recovery was high. However, the melting point of the purified solid was wide and lower than that of the pure solid reported in the literature. Explain as to why i) The percent recovery of the solid was high.

Explanation / Answer

2a)

1) The student must ensure that all the solid is dissolved in hot solvent prior to filtration.

2) If there are impurities which are insoluble, the student must select another solvent to dissolve all.

2b) The percent recovery was high because the student immediately put the flask in the ice bath, which caused a precipitation and no a crystallization (which is what you want). In the precipitate is the solid of interest and impurities and therefore the recovery was high and measuring the melting point a wrong result was obtained.

To obtain reliable results the student should have placed the flask to cool to room temperature first and then in ice bath (to occur crystallization).

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