Q: Which of the following is the correct formula for calculating percent recovery in a recrystallization experiment?

$$\frac{\text{mass of pure product}}{\text{mass of impure sample}} \times 100$$

Question 1

Which of the following statements best explains why sodium benzoate precipitates when the pH of its solution is lowered below its pKa?

Accepted Answer

At low pH, benzoic acid is formed, which is less soluble in water and precipitates out.

Question 2

A mixture contains two compounds: one highly polar and one nonpolar. Which extraction solvent would best separate these compounds?

Accepted Answer

Water and hexane

Question 3

Which of the following is the correct formula for calculating percent recovery in a recrystallization experiment?

Accepted Answer

$$\frac{	ext{mass of pure product}}{	ext{mass of impure sample}} 	imes 100$$

Question 4

During thin layer chromatography (TLC), a student observes that a compound has an $$R_f$$ value of 0.85. What does this indicate about the compound's interaction with the stationary phase?

Accepted Answer

The compound is highly nonpolar and interacts weakly with the stationary phase.

Question 5

Which of the following best describes the effect of impurities on the melting point of a pure organic compound?

Accepted Answer

Impurities lower the melting point and broaden the melting point range.

Question 6

A student separates the components of Panacetin using extraction. Which property is most important for choosing the extraction solvent?

Accepted Answer

The solvent should be immiscible with water and selectively dissolve one component.

Question 7

Which equation correctly calculates the enantiomeric excess (ee) of a mixture containing 70% of the R enantiomer and 30% of the S enantiomer?

Accepted Answer

$$ee = |70\% - 30\%| = 40\%$$

Question 8

In gas chromatography, which factor will most likely increase the retention time of a compound?

Accepted Answer

Increasing the polarity of the stationary phase for a polar compound

Question 9

A student analyzes a mixture using TLC and observes two spots: one with $$R_f = 0.2$$ and one with $$R_f = 0.8. $$Which spot likely corresponds to the more polar compound?

Accepted Answer

The spot with $$R_f = 0.2$$

Question 10

Which of the following is NOT a correct safety practice when performing organic chemistry lab techniques?

Accepted Answer

Heating organic solvents in a closed container

Question 11

A student is asked to identify an unknown compound using melting point determination. The observed melting point is lower and broader than the literature value. What is the most likely explanation?

Accepted Answer

The sample contains impurities.

Question 12

Which of the following statements about fractional distillation is correct?

Accepted Answer

Fractional distillation is used to separate compounds with very similar boiling points.

Question 13

In a TLC experiment, a student uses a very polar solvent as the mobile phase. What effect will this have on the $$R_f$$ values of most compounds?

Accepted Answer

The $$R_f$$ values will increase.

Question 14

Which of the following best describes the role of the stationary phase in chromatography?

Accepted Answer

It interacts with compounds to slow their movement based on polarity.

Question 15

A student is given a mixture of benzoic acid and naphthalene. Which extraction method would best separate these two compounds?

Accepted Answer

Extract with aqueous sodium hydroxide, then acidify the aqueous layer.

Question 16

Which of the following is the correct formula for calculating the retention factor ($$R_f) in $$TLC?

Accepted Answer

$$R_f = \frac{	ext{distance traveled by compound}}{	ext{distance traveled by solvent front}}$$

Question 17

A student is analyzing a mixture using gas chromatography. Which change will most likely decrease the resolution between two closely eluting peaks?

Accepted Answer

Increasing the temperature of the column

Question 18

Which of the following statements about the use of melting point depression to identify mixtures is correct?

Accepted Answer

A mixture of two compounds will have a melting point lower and broader than either pure compound.

Question 19

Which of the following is a correct statement about the use of a drying agent in organic extraction?

Accepted Answer

A drying agent removes water from the organic layer.

Organic Chemistry Lab Exam Study Guide: Techniques, Concepts, and Applications

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