Q: If the mass of tea leaves used is 5.0 g and the mass of crude caffeine obtained is 0.10 g, what is the percentage yield of caffeine?

$$\frac{0.10}{5.0} \times 100\% = 2.0\%$$

Question 1

Which of the following best describes the principle behind the extraction of caffeine from tea leaves using dichloromethane (DCM) and water?

Accepted Answer

Caffeine partitions between water and DCM based on its relative solubility in each solvent, governed by the distribution coefficient $$K_D.$$

Question 2

Given that the distribution coefficient $$K_D$$ for caffeine in a water/DCM system is 7.8, what does this value indicate?

Accepted Answer

Caffeine is 7.8 times more soluble in DCM than in water.

Question 3

During the extraction, why is sodium carbonate ($$\mathrm{Na_2CO_3}) $$added to the tea extract before mixing with DCM?

Accepted Answer

To convert acidic impurities (such as tannins) into their ionic forms, making them less likely to partition into the DCM layer.

Question 4

After shaking the separatory funnel containing the tea extract and DCM, two layers form. How can you determine which layer is DCM?

Accepted Answer

The DCM layer is always the lower layer because it is denser than water.

Question 5

Why is it important to wash the DCM extract with several portions of fresh DCM rather than a single large volume?

Accepted Answer

Multiple small washes increase the total amount of caffeine extracted due to the partitioning equilibrium.

Question 6

Suppose you extracted caffeine from tea leaves using 30 mL of DCM and obtained 9.66 grams of caffeine in the DCM layer, while 0.34 grams remained in the aqueous layer. What is the experimental distribution coefficient $$K_D$$ for caffeine in this system?

Accepted Answer

$$K_D = \frac{9.66}{0.34}$$

Question 7

What is the main reason for performing crystallization after extracting crude caffeine?

Accepted Answer

To purify caffeine by allowing only caffeine to crystallize from a supersaturated solution, leaving impurities in solution.

Question 8

Which of the following hazards is associated with dichloromethane (DCM) and what is the appropriate safety precaution?

Accepted Answer

DCM is a potent carcinogen and inhalation of its vapors can cause dizziness and nausea; always use DCM under a fume hood.

Question 9

If the mass of tea leaves used is 5.0 g and the mass of crude caffeine obtained is 0.10 g, what is the percentage yield of caffeine?

Accepted Answer

$$\frac{0.10}{5.0} 	imes 100\% = 2.0\%$$

Question 10

During the experiment, why is it important to cool the tea extract to room temperature before adding DCM?

Accepted Answer

To avoid excessive evaporation of DCM and ensure safe handling, as DCM is volatile and more hazardous at higher temperatures.

Question 11

A student forgets to add sodium carbonate ($$\mathrm{Na_2CO_3}) $$before extracting with DCM. What is the most likely consequence?

Accepted Answer

Acidic impurities such as tannins may co-extract into the DCM layer, contaminating the crude caffeine.

Question 12

Which of the following best explains why caffeine can be separated from tea leaves using liquid-liquid extraction?

Accepted Answer

Caffeine has different solubilities in water and DCM, allowing it to partition preferentially into the DCM layer.

Question 13

If the theoretical caffeine content in tea leaves is 3% by mass, and you extract 0.12 g of caffeine from 5.0 g of tea leaves, what is your percent recovery relative to the theoretical maximum?

Accepted Answer

$$\frac{0.12}{0.15} 	imes 100\% = 80\%$$

Question 14

Which of the following is NOT a reason to use a separatory funnel in the extraction of caffeine?

Accepted Answer

To provide a closed system for heating the mixture.

Question 15

After extraction, why is the DCM solution dried over sodium sulfate ($$\mathrm{Na_2SO_4}$$)?

Accepted Answer

To remove residual water from the organic layer.

Question 16

Which of the following best describes the process of crystallization as used in this experiment?

Accepted Answer

Crystallization occurs when a solution is cooled or concentrated to the point of supersaturation, causing the pure compound to form solid crystals.

Question 17

A student performs the extraction and finds that the DCM layer is cloudy and does not separate cleanly from the aqueous layer. What is the most likely cause and solution?

Accepted Answer

An emulsion has formed; gently swirl the funnel and allow it to sit undisturbed, or add a small amount of saturated sodium chloride solution to help break the emulsion.

Question 18

Which of the following is a real-world application of the liquid-liquid extraction technique used in this experiment?

Accepted Answer

Extraction of essential oils from plants using organic solvents.

Question 19

If you use 3 separate 20 mL portions of DCM to extract caffeine from an aqueous solution, rather than a single 60 mL portion, what is the expected effect on the total amount of caffeine extracted?

Accepted Answer

The total amount of caffeine extracted will be greater due to repeated partitioning favoring transfer to the DCM.

Extraction of Caffeine from Tea Leaves: Principles, Procedures, and Calculations

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