please help Mid-term Exam CHM250, spring 2018 NAME (print) 1. Recrystallization
ID: 1033535 • Letter: P
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Mid-term Exam CHM250, spring 2018 NAME (print) 1. Recrystallization 1)Why are stemless funnels used instead of long-stem funnels to filter hot solutions through filter papers? (5pts 2)Why is the final product from the crystallization process isolated by vacuum filtration and not by gravity filtration? (S5pts) fluted 3) For recrystallization of 3g impure naphthalene (2.7e pure naphthalene inside), one student get only 0.5g product. Why? Choose 5 reasons which may lead to the low yield, from the 10 choice given to you below. (10pts) A) After mixing of the impure naphthalene and solvent, the heating time is long B) After mixing of the impure naphthalene and solvent, the heating temperature is low C) When doing the gravity filtration, the filtering paper is fluted D) When doing the gravity filtration, the solution is hot E) When drying the solution the CaCl2 was not enough. F) After decanting the mixture, there was some liquid left. G) When doing the suction filtration, the solution was hot. H) When doing the suction filtration, the filter paper is fluted 1) When doing the suction filtration, the vacuum is not good. J) When doing the suction filtration, ether was used to wash the filter cake.Explanation / Answer
1) The stemless funnels are used instead of long stem funnels to filter hot solution through fluted filter papers during the recrystallization. This is becasue with long stemmed funnels, crystals are formed in the stem as the solution is coolig down and blocking up the funnels.
2) The isolation of compound by vacuum filtration is faster than the gravity filtration and the impurity level is removed or reduced in the final product. Most of the solvents are removed by suck drying under vacuum. Thus, the final product from the crystallization process is isolated by vauum filtration rather than the gravity filtration.
3) The following reasons may lead to low yield:
(C), (F), (G), (H), (I),
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