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). 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 ol the solid was formed on the filter paper. The hot filtrate was then immediately cooled by placing the container in an icebath. 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 w as then collcctcd 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 recover) of the solid was high. ii, The melting point range of the purified solid was wide and lower than that of the pure solid as reported in the literat.

Explanation / Answer

a) Percent yield depend upon how much of substance is recovered or obtained from the crude mixture.

In the method described, first not all the solid was dissolved so some part of material is lost here. During filtration due to high concentraion of solute in the solvent, when temperature went down slightly while filtering, solid material precipitated out. This is again considered a loss. Finally all these add up to give a net low percent yield of compound.

b) i) The solution when suddenly cooled, solidification of all the desired compound along with impurities occured, which gave a higher percent yield of the crystallization.

ii) The melting point of pure solid is higher and sharper then when an impurity is added to the pure solid. Solids have intermolecular bonds present in the system which is what we need to break when substance is heated and melted. When an impurity is added, it goes and sits between pure solid molecules. So we have less number of intermolecular bonds in the system now which required lower amount of energy to break and so the melting point is lower.

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