Answer to Question #291940 in Organic Chemistry for thomas

Question

1. In a final very careful and correct recrystallization from acetone, benzoic acid (pure mp 122-122.3°C) was melted on a melting point machine immediately after vacuum filtration and washing, and found to have a melting point of 116-121°C. Why is the melting point not 122-122.3°C? Assume that there are no solid compounds present other than benzoic acid present, and suggest what may be present to affect melting point, and what role that presence does.

2. What are 2 advantages of using gravity filtration over vacuum filtration when removing a solid impurity from a hot mixture? 

Answer 1

1. Due to the presence of impurities in the crude benzoic acid obtained by vacuum filteration, the melting point depression is observed. Depression in melting point indicate the presence of impurity in the compound. Because of the impure compound, the melting point is lower.

2. Gravity filtration is used when removing a solid impurity from a hot mixture over vacuum filteration because a basic filtration under gravity is used when you are moving the solid precipitate to isolate the filtrates, the liquid, for further testing.