Purification Techniques

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MCAT Physical › Purification Techniques

Questions 1 - 10
1

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

Decreasing the atmospheric pressure

Decreasing the vapor pressure

Decreasing the temperature

Decreasing the mole fraction of the substance

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.

2

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

Decreasing the atmospheric pressure

Decreasing the vapor pressure

Decreasing the temperature

Decreasing the mole fraction of the substance

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.

3

In gel electrophoresis, a negatively charged particle will migrate towards the __________ and a positively charged particle will migrate towards the __________.

anode . . . cathode

cathode . . . anode

anode . . . anode

cathode . . . cathode

Explanation

Gel electrophoresis is a technique used to separate molecules based on size or charge. Charged particles can be separated because they migrate towards different ends of the gel.

Negatively charged particles always migrate towards the positive pole whereas positively charged particles always migrate towards the negative pole (opposites attract). In gel electrophoresis, the positive pole is called the anode and the negative pole is called the cathode; therefore, the charged particles will migrate to the respective nodes.

4

In gel electrophoresis, a negatively charged particle will migrate towards the __________ and a positively charged particle will migrate towards the __________.

anode . . . cathode

cathode . . . anode

anode . . . anode

cathode . . . cathode

Explanation

Gel electrophoresis is a technique used to separate molecules based on size or charge. Charged particles can be separated because they migrate towards different ends of the gel.

Negatively charged particles always migrate towards the positive pole whereas positively charged particles always migrate towards the negative pole (opposites attract). In gel electrophoresis, the positive pole is called the anode and the negative pole is called the cathode; therefore, the charged particles will migrate to the respective nodes.

5

After performing a TLC experiment, a researcher determines the Rf value of a component to be 2. He also notes that the solvent travelled a distance of 4cm on the plate. What can you conclude about this experiment?

The reported data does not seem valid

The component travelled a distance of 8cm

The solvent is not volatile

There are multiple components in the mixture

Explanation

To solve this problem we need to recall the definition of Rf.

The solvent is typically the most mobile substance on a TLC plate; therefore, it travels the farthest distance. This means that Rf is always less than one because the numerator of Rf is always smaller than the denominator. The question states that the Rf value is 2. Since this is greater than 1, the results don’t seem valid. This value indicates that the solute compounds were able to teravel farther on the TLC plate than the solvent (mobile phase).

6

After performing a TLC experiment, a researcher determines the Rf value of a component to be 2. He also notes that the solvent travelled a distance of 4cm on the plate. What can you conclude about this experiment?

The reported data does not seem valid

The component travelled a distance of 8cm

The solvent is not volatile

There are multiple components in the mixture

Explanation

To solve this problem we need to recall the definition of Rf.

The solvent is typically the most mobile substance on a TLC plate; therefore, it travels the farthest distance. This means that Rf is always less than one because the numerator of Rf is always smaller than the denominator. The question states that the Rf value is 2. Since this is greater than 1, the results don’t seem valid. This value indicates that the solute compounds were able to teravel farther on the TLC plate than the solvent (mobile phase).

7

Which of the following is true regarding sodium dodecyl sulfate (SDS)?

It allows for separation of proteins by size

It binds to the primary structure of proteins

It adds positive charge to proteins

It preserves the mass-to-charge ratio of proteins

Explanation

SDS is a substance added to polyacrylamide gels to separate substances based on size. This method is called SDS-PAGE. SDS binds to the secondary structure of proteins and gives them an overall negative charge; the bigger the substance the bigger the negative charge. The larger mass compensates for the bigger charge; therefore, SDS makes it so that the mass-to-charge ratio of all substances is the same. This standardizes the electric force experienced by each molecule, making size the only driving force for migration through the gel.

8

Which of the following is true regarding sodium dodecyl sulfate (SDS)?

It allows for separation of proteins by size

It binds to the primary structure of proteins

It adds positive charge to proteins

It preserves the mass-to-charge ratio of proteins

Explanation

SDS is a substance added to polyacrylamide gels to separate substances based on size. This method is called SDS-PAGE. SDS binds to the secondary structure of proteins and gives them an overall negative charge; the bigger the substance the bigger the negative charge. The larger mass compensates for the bigger charge; therefore, SDS makes it so that the mass-to-charge ratio of all substances is the same. This standardizes the electric force experienced by each molecule, making size the only driving force for migration through the gel.

9

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

simple . . . fractional

simple . . . simple

fractional . . . fractional

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.

10

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

simple . . . fractional

simple . . . simple

fractional . . . fractional

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.

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