Answers and Explanations

DISCRETE PRACTICE QUESTIONS

1 . B

Fractional  distillation    is  the most    effective   procedure   for separating  two liquids that    boil    within  a

few degrees of  each    other.  Ethyl   acetate and ethanol boil    well    within  25°C    of  each    other   and thus

would   be  good    candidates  for fractional  distillation.   Fractional  distillation    could   also    be  used    for

the liquids in  choice  (C),    but would   require lower   pressures   because of  their   high    boiling points.

2 . C
By extracting with sodium hydroxide, benzoic acid will be converted to its sodium salt, sodium
benzoate. Sodium benzoate, unlike its conjugate acid, will dissolve in an aqueous solution. The
aqueous layer simply has to be acidified afterward to retrieve benzoic acid. Choice (A) is incorrect
because diethyl ether and tetrahydrofuran are both nonpolar and are miscible. Hydrochloric acid
will not transform benzoic acid into a soluble salt, so choice (B) is incorrect. Finally, choice (D) is
incorrect because protonated benzoic acid has limited solubility in water.

3 . D
It is more effective to perform four successive extractions with small amounts of ether than to
perform one extraction with a large amount of ether.

4 . D
Hexane is less polar than ether and, therefore, is less likely to displace polar compounds
adsorbed to the silica gel. This would decrease the distance these polar compounds would travel,
decreasing Rf values.

5 . D
In column chromatography, as in TLC, the less polar compound travels most rapidly. This means
that 1-naphthalenemethanol,with the highest Rf value, would travel most rapidly and would be
the first to elute from the column.