MCAT Separations Practice Questions: Extraction, Distillation and Chromatography

Answer: A. 0.60; compound Q is most polar

Rf = spot distance / solvent front distance = 4.8/8.0 = 0.60. On polar silica, the most polar compound binds the stationary phase most strongly and travels least, so compound Q (Rf 0.25) is most polar. The 1.67 choice inverts the ratio, which is impossible since Rf cannot exceed 1, and 0.40 is 1 minus the correct value.

Answer: B. The carboxylic acid only

Bicarbonate can deprotonate an acid whose pKa is well below 6.4, so the carboxylic acid becomes a water-soluble carboxylate salt. The phenol, with a pKa near 10, is too weak an acid to be deprotonated by bicarbonate and stays neutral in the ether. A stronger base such as sodium hydroxide would be needed to pull the phenol out too.

Answer: C. Dilute hydrochloric acid

Acid protonates the basic amine to form an ammonium salt, which is charged and dissolves in water, while the ketone stays in the ether. Basic washes such as sodium hydroxide or bicarbonate leave the amine neutral and organic-soluble. Brine mainly removes dissolved water from the organic layer and does not react with either compound.

Answer: D. In the lower layer, because dichloromethane is denser than water

The nonpolar product prefers the organic solvent, and layer position depends only on density. Dichloromethane, at about 1.33 g/mL, is denser than water, so it forms the bottom layer. Choice A is a common trap based on solvents like diethyl ether, which is less dense than water and floats.

Answer: C. Boiling points of 72 °C and 84 °C

Simple distillation separates liquids well only when their boiling points differ by more than about 25 °C. The 72 and 84 °C pair differs by just 12 °C, so it needs a fractionating column, which provides many vaporization and condensation cycles. The other pairs differ by more than 100 °C and separate well by simple distillation.

Answer: A. Distill it under reduced pressure, which lowers its boiling point

A liquid boils when its vapor pressure equals the external pressure, so lowering the pressure lets it boil at a lower temperature, below the point where it decomposes. A fractionating column improves separation but does not lower the boiling point, and raising the pressure or heating faster would push the temperature even higher.

Answer: D. It dissolves the product readily at room temperature

Recrystallization depends on a steep solubility difference: the product must dissolve when hot and crystallize when cold. A solvent that dissolves the product readily at room temperature would keep much of it in solution and give a poor recovery. Keeping impurities dissolved in the cold mother liquor is exactly how the method purifies the product.

Answer: B. The unknown is Y, because mixing with Y did not lower or broaden the melting range

Mixing two different compounds acts like adding an impurity, which lowers and broadens the melting range, as seen with X. Mixing a compound with more of itself changes nothing, as seen with Y, so the unknown is Y. The sharp melting of the unknown alone argues against it being a mixture, and the mixed test solves the problem that the pure melting points match.

Answer: A. Hydrocarbon, ester, carboxylic acid

Silica is polar, so polar compounds adsorb strongly and move slowly in a nonpolar eluent. The hydrocarbon interacts least and elutes first, the moderately polar ester comes next, and the carboxylic acid, which hydrogen bonds strongly with silica, elutes last. Choice B is the order expected on a nonpolar, reverse-phase column.

Answer: C. The 98 °C alkane elutes first, because it is the most volatile and travels mostly with the carrier gas

In gas chromatography, compounds partition between the moving gas and the stationary liquid coating. With similar polarity, the most volatile compound, the one with the lowest boiling point, is carried along by the gas for the largest share of its transit and reaches the detector first. The carrier gas is inert, but the stationary phase still separates the compounds, and flow rate shifts all retention times without reversing their order.