MCAT Physical : Distillation

Study concepts, example questions & explanations for MCAT Physical

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Example Questions

Example Question #1 : Distillation

Consider the following solutions.

Solution A: 1M sodium chloride solution

Solution B: 1M calcium nitrate solution

Solution C: 1M sucrose solution

Equal volumes of the solutions are combined and the mixture is added to a distillation column. Which of the following solutions will separate first?

Possible Answers:

These solutions cannot be separated via distillation

Solution C

Solution A

Solution B

Correct answer:

Solution C

Explanation:

Distillation is a process of separating a liquid from solutes or other liquids. It utilizes the boiling point differences to separate substances. A substance with a low boiling point will evaporate first in a distillation column and will be isolated first. The question is asking which solution will be isolated first; therefore, we need to figure out which solution has the lowest boiling point. Recall that the boiling point of a solution is elevated when there are more solutes present in the solution. Sodium chloride () contributes two solutes (sodium ions and chloride ions). Calcium nitrate () contributes three solutes (one calcium ion and two nitrate ions). Sucrose does not dissociate into ions in solution; therefore, it only contributes one solute. This means that the sucrose solution will have the lowest amount of molecules in solution, the lowest boiling point, and will be separated first.

Example Question #2 : Distillation

Small differences in boiling point require the use of __________ distillation and large differences in boiling point require the use of __________ distillation. 

Possible Answers:

fractional . . . simple

fractional . . . fractional

simple . . . fractional

simple . . . simple

Correct answer:

fractional . . . simple

Explanation:

There are two types of distillation. Simple distillation is used to separate molecules that have very different boiling points. Fractional distillation is used to separate molecules with small differences in boiling points. Fractional distillation is often used if the difference between boiling points is less than . In simple distillation, the vapor is immediately collected in a condenser. On the other hand, fractional distillation allows vapor to condense and revaporize several times. These repeated cycles allow fractional distillation to purify the vapor better than simple distillation.

Example Question #3 : Distillation

Which of the following mixtures can be separated using fractional distillation (boiling points of each substance given in )? 

I. Chloroform (62.2) and  (76.7)

II. Iodine (184.3) and mercury (356.9)

III. Nitric acid (120) and sulfuric acid (310)

Possible Answers:

II and III

II only

I, II, and III

I only

Correct answer:

I, II, and III

Explanation:

Distillation is used to separate molecules with different boiling points. Simple distillation is used to separate molecules with vastly different boiling points. Fractional distillation, on the other hand, is a refined form of simple distillation that can be used to separate molecules with similar boiling points. Note that fractional distillation can separate molecules with either different or similar boiling points; therefore, fractional distillation can be used to separate any of the given mixtures. 

Example Question #4 : Distillation

Which of the following conditions will result in the greatest increase in the rate of distillation of a substance? 

Possible Answers:

Decreasing the mole fraction of the substance

Decreasing the atmospheric pressure

Decreasing the temperature

Decreasing the vapor pressure

Correct answer:

Decreasing the atmospheric pressure

Explanation:

Rate of distillation is increased when the ability of a substance to become a vapor is increased. Recall that vapor is created when enough heat is applied to the liquid. The temperature at which the liquid becomes vapor is called the boiling point. A liquid turns into a vapor when the vapor pressure (pressure applied by the vapor from the liquid) equals the atmospheric pressure. Decreasing the atmospheric pressure will make it easier for the liquid to turn into a vapor; therefore, this will increase the rate of distillation.

Decreasing the vapor pressure will remove vapor from system. This will make it harder to distill substances. Decreasing temperature will move the system away from the boiling point, thereby decreasing the amount of vapor. Decreasing mole fraction of the substance will decrease the surface area of the substance (at the surface of the solution). Liquid molecules need to be present at the surface to escape the solution and become vapor; therefore, decreasing mole fraction will decrease the amount of vapor.

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