Question

1) which one is best for crystallization, 2) the solvent in which the compound (solid) is too soluble, and 3) the solvent in which the compound is not sufficiently soluble?

A. Phenanthrene; toluene, 95% ethyl alcohol, water
B. Cholesterol; ether, 95% ethyl alcohol, water
C. Acetaminophen; toluene, 95% ethyl alcohol, water
D. Urea; hexane, 95% ethyl alcohol, water

Answer 1

Take note of whether or not the molecule is polar. Is it a hydrocarbon - only hydrogen and carbon - and therefore nonpolar? Then take a look at the given solvents. With phenanthrene, you're dealing with a pure hydrocarbon, that means nonpolarity. Toluene has a similar structure - composed on only C & H - therefore it will be too soluble at room temperature and will not make a good solvent here. Water is highly polar; those are its only components. Therefore, most-likely, water will not dissolve phenanthrene, even at high temperatures. The final choice is ethyl alcohol, or ethanol. Think of its composition. It has a polar O attached to two ethyl (C2H5) groups. It therefore has both polar and nonpolar attributes. The nonpolar parts will interact with the phenanthrene at room temperatures to make it partially soluble, and at higher temperatures, the polar O will also be able to interact with the compound, thus increasing the solubility. That is the key. But that lengthy explanation was if you really want to understand the process. It helps if you've had to actually do this in the lab. But, if you just want the answers, here they are: Cholesterol - ethanol, Acetaminophen - water, Urea - ethanol.

So finally answer is (A) .