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MCAT Sample Questions : Biological Sciences

Passage VI

Carvone (shown below) can exist as either of two enantiomers.One of them, (+)-carvone, provides the characteristic odor in caraway seed oil, whereas (-)-carvone is responsible for the fragrance of spearmint oil.

A chemist attempted to isolate (+)-carvone from caraway seed oil, which consists almost entirely of (+)-carvone and limonene (shown below).

Because the two compounds have very different boiling points, the chemist decided to separate them by vacuum fractional distillation.The apparatus in Figure 1 was assembled, and a sample of caraway seed oil was placed in the distillation flask.An ebulliator was lowered into the distillation flask to introduce small air bubbles into the system.The fractionating column and distillation head were wrapped with glass wool, and the apparatus was connected with thick tubing to a vacuum source.The contents of the distillation flask were heated, and two fractions were collected in the receiving flask.

Figure 1 Vacuum fractional distillation apparatus

Following are some sample questions on this passage:

  1. When the caraway seed oil is heated, which of the two components will most likely distill first?

    1. Limonene, because it has the lower boiling point
    2. Limonene, because it has the higher boiling point
    3. (+)-Carvone, because it has the lower boiling point
    4. (+)-Carvone, because it has the higher boiling point

    Answer: A

    Explanation: The carbonyl group (C=O) in carvone makes carvone more polar and has a higher boiling point than the hydrocarbon limonene.Thus, answer choice A is the best answer.

  2. The most likely function of the ebulliator is to:

    1. keep the condensed vapors cool in the receiving flask.
    2. promote the establishment of a high vacuum in the system.
    3. prevent superheating of the liquid to be distilled.
    4. provide an outlet when the pressure inside the system becomes too high.

    Answer: C

    Explanation: The passage mentions that the ebulliator was added to introduce small air bubbles into the system.This is the same function provided by a boiling chip at atmospheric pressure.The air bubbles break the surface tension of the liquid being heated and prevent superheating and bumping.Thus, answer choice C is the best answer.

  3. Which of the following experimental modifications will most likely improve the degree of separation between limonene and (+)-carvone?

    1. Heating the distillation flask at a slower rate
    2. Using a vacuum source that can achieve a lower pressure inside the distillation apparatus
    3. Cooling the condenser with ice water
    4. Using a shorter fractionating column

    Answer: A

    Explanation: We presume that the separation of limonene and (+)-carvone can be improved by modifying either the apparatus shown in Figure 1 or the procedure (i.e., the separation as described in the passage did not achieve a totally satisfactory separation.) The separation of the two liquids takes place in the fractionating column as the two liquids vaporize and condense, with the lower-boiling liquid distilling first.If the fractionating column is shortened (Answer D), the liquids will vaporize and condense fewer times (i.e., there will be fewer theoretical plates) and the degree of separation will worsen.Cooling the condenser with ice water (Answer C) will have no effect on the degree of separation because condensation takes place after the separation has occurred.Creating a lower pressure inside the distilling apparatus (Answer B) will lower the boiling points of both liquids and narrow their difference in boiling point, making it necessary to also increase the length of the fractionating column in order to achieve the same degree of separation as obtained originally.Lengthening the fractionating column was not part of answer choice B.Heating the distillation flask (i.e., increasing the temperature) at a slower rate (Answer A) will allow both liquids more time in the fractionating column (increase the number of theoretical plates, allowing liquid and vapor to equilibrate); therefore, of the four options, this one is most likely to improve the degree of separation of the two compounds.Thus, answer choice A is the best answer.

  4. Carvone is shown below with certain carbon atoms labeled.

    (+)-Carvone and ()-carvone differ in the orientations of the substituents around which of the following carbon atoms?

    1. Carbon 2 only
    2. Carbon 5 only
    3. Carbons 2 and 5 only
    4. Carbons 2, 5, and 7 only

    Answer: B

    Explanation: Carbon 5 is the only stereogenic carbon atom (chirality center); carbons 2 and 7 are not stereocenters.Thus, answer choice B is the best answer.

  5. If a leak develops in the vacuum distillation apparatus, the boiling points of the two components of caraway seed oil will:

    1. both increase.
    2. both decrease.
    3. both remain the same.
    4. become more similar.

    Answer: A

    Explanation: The boiling point of a liquid is the temperature at which the vapor pressure of the liquid equals the surface pressure.The normal boiling point is measured at 1 atm pressure.The vapor pressure of a liquid increases with increasing temperature.Hence, the boiling point of a liquid decreases as the pressure on the surface of the liquid is decreased.If a leak develops in the apparatus, the surface pressure will increase, as will the boiling points of both liquids.Thus, answer choice A is the best answer.

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