MCAT Biology : Organic Chemistry, Biochemistry, and Metabolism

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

Example Question #11 : Mcat Biological Sciences

Which of the following mixtures would not be adequately separated via distillation?

Possible Answers:

A solution of polar and nonpolar compounds

A solution containing three diastereomers

A solution of alcohols of different sizes

Racemic mixture

Correct answer:

Racemic mixture

Explanation:

Distillation involves the separation of compounds by taking advantage of their different boiling points. Mixtures with similar boiling points would not be adequately separated using distillation.

A racemic mixture is a mixture of enantiomers, which have identical boiling points. As a result, distillation would not be effective for separating a racemic mixture.

Diastereomers will have differing physical properties and can be separated by boiling point. Larger alkane chains result in higher boiling points, allowing separation of different sized alcohols by distillation. Polar compounds will have greater intermolecular forces, generally leading to higher boiling points, and allowing them to be separated from nonpolar compounds.

Example Question #11 : Organic Analyses And Lab Techniques

Which of the following statements about recrystallization is true?

Possible Answers:

When a compound is recrystallized the desired product crashes out of solution, while the impurities remain dissolved

The choice of solvent is completely interchangeable for any recrystallization

When a compound is recrystallized the impurities crash out of the solution, while the desired product remains dissolved

Temperature plays no role in recrystallization techniques

Correct answer:

When a compound is recrystallized the desired product crashes out of solution, while the impurities remain dissolved

Explanation:

During recrystallization, a compound is separated from impurities based on differences in solubility in a given solvent.

To correctly recrystallize a compound, the correct solvent must be used. Good solvents are ones in which the product is slightly soluble and impurities are completely soluble. Additionally, the temperature plays a large role in this process, as you must decrease the temperature of the solution to decrease the solubility of desired product enough to have the product crash out as a solid.

Example Question #12 : Organic Analyses And Lab Techniques

You are heating up a crude sample of potassium chloride to purify for use in an experiment. Which of the following methods provides the quickest and purest crystal product?

Possible Answers:

Fast melting with slow recrystallization

Slow melting with fast recrystallization

Slow melting with slow recrystallization

Fast melting with fast recrystallization

Multiple rounds of slow melting with quick recrystallization

Correct answer:

Fast melting with slow recrystallization

Explanation:

Recrystallization is the purification of a product based on differences in solubility. The desired compound is placed in a solvent in which it is just barely soluble, but in which impurities are highly soluble. The mixture is heated to melt the desired compound, creating a homogeneous solution. The solution is then cooled, allowing the desired compound to slowly precipitate out of solution due to its low solubility.

For the purest product, the most important aspect is to let your product cool down as slowly as possible so that the crystal structure contains the least amount of impurities. Melting the product slowly or quickly will not affect the purity of the product, and thus a faster melting stage will allow for the quickest and purest product formation.

Example Question #12 : Mcat Biological Sciences

A researcher is trying to choose an appropriate solvent to recrystallize his product. He first adds a few drops of a solvent at room temperature and the crystals do not begin to dissolve. Is this potentially a good choice of solvent?

Possible Answers:

Maybe; the researcher should heat the sample to see if the crystals dissolve at the boiling point of the solvent and recrystallize when cooled

Maybe; the researcher should wait to see if the crystals dissolve after a long period of time

Yes; the solvent should not dissolve the crystals at room temperature

No; the solvent should always dissolve the crystals at room temperature

Correct answer:

Maybe; the researcher should heat the sample to see if the crystals dissolve at the boiling point of the solvent and recrystallize when cooled

Explanation:

When choosing an appropriate solvent for recrystallization, it is crucial that the crystals do not dissolve at room temperature and do dissolve when the sample is heated. For this reason, we cannot say without further information whether or not this solvent will be useful. For example, the researcher could heat the sample to the boiling point of the solvent only to discover that his crystals still do not dissolve. The researcher first has to heat the sample and observe the effect on the crystals to know whether or not this solvent is useful.

Example Question #12 : Mcat Biological Sciences

Chromatography is a technique used to separate compounds that are in solution with one another. In paper chromatography, a piece of paper is dotted with the sample mixture near the bottom and then placed in a liquid solvent. The solvent then dissolves the sample and separates the compounds as it moves up the paper.

Which of the following statements is true concerning paper chromatography?

Possible Answers:

More massive compounds will be lower on the paper compared to less massive compounds.

Less massive compounds will be lower on the paper compared to more massive compounds.

Polar compounds will be higher on the paper compared to nonpolar compounds.

Nonpolar compounds will be higher on the paper compared to polar compounds.

Correct answer:

Nonpolar compounds will be higher on the paper compared to polar compounds.

Explanation:

Paper chromatography is used to separate compounds based on differing polarities. Polar compounds will move more slowly, because they have a higher affinity for the polar paper strip. As a result, they will stop at a lower spot on the paper compared to the nonpolar compounds, which will end up near the top of the strip. In some cases, a non-polar paper can be used, but polar paper is standard and should be assumed if no other information is given.

Example Question #13 : Mcat Biological Sciences

Chromatography is a technique used to separate compounds that are in solution with one another. In paper chromatography, a piece of paper is dotted with the sample mixture near the bottom and then placed in a liquid solvent. The solvent then dissolves the sample, and separates the compounds as it moves up the paper. 

Each component of the mixture will have an Rf factor. This is calculated by dividing the distance the compound traveled on the paper by the distance the solvent traveled on the paper. 

 

Which of the following compounds would you predict to have the largest Rf factor?

Possible Answers:

Acetone

Serine

Glucose

Hexane

Correct answer:

Hexane

Explanation:

In paper chromatography, the most nonpolar compound will travel the farthest on the paper strip. The compound that travels the farthest will have the largest Rf  factor. Hexane is nonpolar, so it will travel the farthest and have the largest Rf factor out of these possible compounds.

Example Question #13 : Organic Analyses And Lab Techniques

 

 

Thin layer chromatography (TLC) is used to separate and identify mixture components based on polarity. The Rf value is used to characterize the mixture components by describing the distance traveled by the mixture components relative to the mobile phase. Which of the following mixture components would have the greatest Rf value if the mobile phase is hexane?

Possible Answers:

Benzene

Dimethyl ether

Acetic Acid

Benzyl alcohol

Benzoic Acid

Correct answer:

Benzene

Explanation:

The greatest Rf value is associated with the least polar mixture component. This is because the component with the greatest solubility in the mobile phase will be pulled by the solvent upwards, and will not be attracted to the polar stationary phase (the paper). More polar solutes will travel more slowly due to attractive forces with the polar stationary phase. Out of all of the answer choices, benzene is least polar and is therefore the correct answer.

Example Question #1 : Chromatography

Which of the following functional groups would be expected to have the largest  value during a thin-layer chromatography (TLC) experiment with an ether solvent?

Possible Answers:

Alcohol

Amine

Alkane

Halide

Correct answer:

Alkane

Explanation:

The value is proportional to the affinity of the solute to the solvent. The solvent acts as the mobile phase along a polar paper stationary phase. Polar compounds will interact more with the paper, travelling slowly, while nonpolar compounds will interact more with the solvent, travelling more quickly.

A large  value represents a large proclivity for the mobile solvent in the experiment. Because we are using ether, a non-polar solvent, we would expect non-polar compounds to travel the farthest on our plate. Of the answer choices, alkanes are the least polar and would thus travel farthest into the mobile phase of the four functional groups. A polar functional group, like a halide, will interact more in the stationary phase, and will thus has a significantly smaller  value. 

Example Question #1 : Chromatography

Which of the following purification techniques would best separate a nonpolar solute from a polar solute?

Possible Answers:

Distillation

Ion exchange chromatography

Thin layer chromatography

Mass spectroscopy

Correct answer:

Thin layer chromatography

Explanation:

Generally, extraction is the best means of separating two solutes based on polarity. This technique allows separation based on solubility in two different solvents, which separate based on polarity.

Extraction, however, is not offered as an answer. The next best option would be thin layer chromatography. In this process, a polar stationary phase is introduced to a nonpolar solvent. Solutes are placed on the stationary phase. The nonpolar solvent acts as the mobile phase. Nonpolar solvents interact more with the mobile solvent, travelling quickly along the polar stationary phase, while polar solutes are attracted to the stationary phase and travel more slowly. This property allows for separation based on polarity.

Ion exchange chromatography is used to separate compounds with different charges, not necessarily differing polarities. Mass spectroscopy will identify compounds based on mass, and distillation will allow for separation based on differences in boiling point and vapor pressure.

Example Question #1 : Chromatography

Chromatography involves the separation of a mixture by allowing a mobile phase to travel along a stationary phase. In thin layer chromatography (TLC), a liquid solution is able to travel along a stationary plate. The distance that a particular compound travels compared to another compound can be determined by comparing the Rf factors for each compound. The Rf factor is determined by dividing the compound's distance by the total distance of the solvent.

Which of the following compounds would have the smallest Rf factor in a standard thin-layer chromatography (TLC) experiment?

Possible Answers:

Butanol

Propane

Hexane

Glucose

Correct answer:

Glucose

Explanation:

The stationary phase in chromatography is typically attracted to the more polar compounds in a solution, while the mobile phase carried the nonpolar compounds. As a result, more polar compounds will move a shorter distance, resulting in a lower Rf factor. Glucose is a very polar molecule, and would move a shorter distance compared to the other options.

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