MCAT Organic Chemistry Review

Separations and Purifications

Practice Questions

1.    Fractional distillation under atmospheric pressure would most likely be used to separate which of the following compounds?

1.    Methylene chloride (boiling point of 40°C) and water (boiling point of 10°C)

2.    Ethyl acetate (boiling point of 77°C) and ethanol (boiling point of 80°C)

3.    Aniline (boiling point of 184°C) and benzyl alcohol (boiling point of 205°C)

4.    Aniline (boiling point of 184°C) and water (boiling point of 100°C)

2.    Which of the following compounds would be most effective in extracting benzoic acid from a diethyl ether solution?

1.    Tetrahydrofuran

2.    Aqueous hydrochloric acid

3.    Aqueous sodium hydroxide

4.    Water

3.    What would be the effect on the Rf values if the thin-layer chromatography (TLC) described below were run with hexane rather than ether as the eluent?

Compound

Retardation Factor in Ether

Benzyl alcohol

0.10

Benzyl acetate

0.26

p-Nitrophenol

0.23

1-Naphthalenemethanol

0.40

1.    No effect

2.    Increase tenfold

3.    Double

4.    Decrease

4.    If benzyl alcohol, benzyl acetate, p-nitrophenol, and naphthalene were separated by column chromatography with ether on silica gel, which compound would elute first according to the Rf values from question 3?

1.    Benzyl alcohol

2.    Benzyl acetate

3.    p-Nitrophenol

4.    1-Naphthalenemethanol

5.    Which of the following would be the best procedure for extracting acetaldehyde from an aqueous solution?

1.    A single extraction with 100 mL of ether

2.    Two successive extractions with 50 mL portions of ether

3.    Three successive extractions with 33.3 mL portions of ether

4.    Four successive extractions with 25 mL portions of ether

6.    Four compounds, I, II, III, and IV, are separated by chromatographic techniques. Compound III is the most polar, II the least polar, and I and IV have intermediate polarity. The solvent system is 85:15 ethanol:methylene chloride. Which spot on the card below likely belongs to compound III?

1.    A

2.    B

3.    C

4.    D

7.    Suppose an extraction with methylene chloride  is performed, with the desired compound initially in brine  In a separatory funnel, which layer will be the organic layer?

1.    Top layer

2.    Bottom layer

3.    No layers are observed; methylene chloride and brine are completely miscible.

4.    More information is needed to answer the question.

8.    Silica gel is often used in thin-layer chromatography. What property does silica gel probably possess that makes it useful for this purpose?

1.    Acidity

2.    Polarity

3.    Specifically sized pores

4.    Aqueous solubility

9.    A mixture of sand, benzoic acid, and naphthalene in ether is best separated by:

1.    filtration, followed by acidic extraction, followed by recrystallization.

2.    filtration, followed by basic extraction, followed by evaporation.

3.    extraction, followed by distillation, followed by gas chromatography.

4.    filtration, followed by size-exclusion column chromatography, followed by extraction.

10.Simple distillation could be used to separate which of the following compounds?

1.    Toluene (boiling point of 111°C) and water (boiling point of 100°C)

2.    Naphthalene (boiling point of 218°C) and butyric acid (boiling point of 163°C)

3.    Propionaldehyde (boiling point of 50°C) and acetic acid (boiling point of 119°C)

4.    Benzene (boiling point of 80°C) and isopropyl alcohol (boiling point of 83°C)

11.In order to separate a biological effector from solution, which chromatographic technique would be the most effective?

1.    Thin-layer chromatography

2.    Ion-exchange chromatography

3.    Affinity chromatography

4.    Size-exclusion chromatography

12.Given a solution of insulin (molecular weight = 5.8 kD) and titin (molecular weight = 3816 kD), which chromatographic technique would be the most effective for separating out usable molecules of titin?

1.    Thin-layer chromatography

2.    Ion-exchange chromatography

3.    Affinity chromatography

4.    Size-exclusion chromatography

13.The gas eluent in gas chromatography and the liquid eluent in paper chromatography are examples of which component of these systems?

1.    Stationary phase

2.    Mobile phase

3.    Column

4.    Fraction

14.During gravity filtration, a student forgets to heat the solution before running it through the filter. After capturing the filtrate, the student analyzes the sample via infrared (IR) spectroscopy and finds none of the desired product in the filtrate. What likely occurred to the student’s product?

1.    The product degraded because of a prolonged filtration time.

2.    The product evaporated with collection of the filtrate.

3.    The product precipitated and is present in the residue.

4.    The product was dissolved in the solvent.

15.Lactoferrin, a milk protein, is a valuable antimicrobial agent that is extracted from pasteurized, defatted milk utilizing a column containing a charged resin. This is an example of which of the following chromatographic techniques?

1.    Thin-layer chromatography

2.    Ion-exchange chromatography

3.    Affinity chromatography

4.    Size-exclusion chromatography

PRACTICE QUESTIONS