Lab Exam 1

Question 1

What procedures may lead to formation of crystals after a solid is dissolved in a solvent?

Answer 1

Cooling the liquid
Partial evaporation of the solvent
Addition of a seed crystal
Addition of a solvent in which the compound has low solubility
Scraping the inside of the flask with a glass rod

Question 2

London Dispersion Forces ____

Answer 2

Are weak compared to ionic interactions
Occur between non-polar compounds
Occur between polar compounds
Are attractive interactions between molecules

Question 3

Is it advantageous for the purity of the product of a recrystallization if the solution containing the recrystallizing solid is cooled slowly?

Answer 3

Yes, because the slow crystallization will make it less likely that the impurities will be entrapped in the growing crystal. It does not lead to a higher yield.

Question 4

What steps lead to a maximized yield of product?

Answer 4

Use of a minimum amount of solvent
Using ice to further cool the crystallizing solid

Question 5

How do you deal with colored impurities?

Answer 5

Add activated charcoal. It works because it has a high surface area with an affinity for attaching to impurities. Pour the solution through a gravity filtration quickly to prevent crystallization in the filter.

Question 6

Why is vacuum filtration preferred for isolation a crystalline material from a liquid?

Answer 6

It actively pulls the mother liquor into the flask and continue pulling a stream of air through, wicking away excess solvent from the crystals so that it has less chance of redissolving any material and any dissolved impurities in the solvent will not be allowed to sit on the surface of the crystal.