lab report chemistry
Lab Report Guidlines:
“Writing lab reports follows a straightforward and structured procedure. It is important to recognize that each part of a lab report is important, so take the time to complete each carefully. A lab report is broken down into eight sections: title, abstract, introduction, methods and materials, results, discussion, conclusion, and references.” Please adhere to the format below when preparing your reports.
The title of the lab report should be descriptive of the experiment and reflect what the experiment analyzed.Ex: “Determining the Free Chlorine Content of Pool Water”
When writing an abstract, try to answer these questions:Why was the research done or experiment conducted?What problem is being addressed?What results were found?What are the meaning of the results?How is the problem better understood now than before, if at all?
(IV) Methods and Materials
The methods and materials section provides an overview of any equipment, apparatus, or other substances used in the experiment, as well as the steps taken during the experiment. If using any specific amounts of materials, make sure the amount is listed.Ex: pipette, graduated cylinder, 1.13mg of Na, 0.67mg Ag
(V) Data & Results
If using graphs, charts, or other figures, present them in the results section of the lab report.Tables should be labeled numerically, as “Table 1”, “Table 2”, etc. Other figures should be labeled numerically as “Figure 1”, “Figure 2”, etc.
When writing a discussion, try to answer these questions:What do the results indicate?What is the significance of the results?Are there any gaps in knowledge?Are there any new questions that have been raised?
(VIII) References (if applicable)
Extraction of Caffeine from Black Tea
Caffeine is a naturally occurring alkaloid produced by tea and coffee shrubs. It is a CNS
stimulant that is believed to act by serving as an antagonist of adenosine receptors on
Caffeine is soluble in hot water and is extracted from coffee grounds or tea leaves when
these products are brewed. While caffeine is water soluble, it is much more soluble in the
organic solvent methylene chloride (CH2Cl2). Methylene chloride is immiscible with water
and when mixed separates from water to form a two-layer mixture. Because methylene
chloride is denser than water it usually comprises the lower layer in the two-part mixture. By
mixing brewed tea with methylene chloride, the caffeine can be extracted into the organic
layer. Since the organic layer is immiscible with water, it can be removed after it separates from the water, and the
solvent evaporated to give nearly pure caffeine.
Preparation of the Brewed Tea
1. Obtain two tea bags. Weigh the tea bags and record the combined mass. The instructor will provide the average
mass of the empty paper bag so that you can determine the mass of the contents.
2. Place both bags into a 150 mL beaker and add enough deionized water to completely cover the tea bags.
3. Cover the beaker with a watch glass and heat it on a hotplate until the water is almost boiling.
4. Heat for 15 minutes and occasionally push down on the tea bag with a test tube to keep it as fully submerged in
water as possible.
5. Be gentle with this step—don’t break the bag! If excessive evaporation of water occurs, add more DI water as
6. Transfer the brewed tea to two plastic centrifuge tubes using a plastic pipette. Try to keep the liquid in each
centrifuge tube equal.
7. Gently press against the tea bag (while it is against the wall of the beaker) with a test tube to squeeze out as much
tea solution as possible.
8. Pour 2 mL of hot water over the tea bag while it is sitting on the bottom of the beaker, again pressing gently
against the bag with a test tube to release as much tea as possible.
9. Divide this additional liquid equally across the two centrifuge tubes.
10. Add 0.5 g of sodium carbonate (Na2CO3) to each tube. Cap the tubes and shake until the solid dissolves. (* Note:
Carefully vent the tubes!)
Extraction of the Caffeine from the Tea
1. When the tea has cooled to room temperature, add 3 mL of methylene chloride (CH2Cl2) to each centrifuge tube.
2. Cap the tubes and gently shake for several seconds, carefully vent the tubes to release any pressure buildup by
slowly opening the caps. Recap the tubes and shake for about 30 seconds (vent the tube occasionally).
a. An emulsion will likely form during the extraction process. To “break” the emulsion, spin the tubes in
the centrifuge for 2-3 minutes. Make sure the centrifuge is balanced with tubes of nearly equal mass
opposite of each other in the rotor. The mixture should have two layers—a nearly colorless bottom
layer and a dark upper layer. If a third frothy green-brown layer is in between the upper and lower
layers, the emulsion is still present, and the tubes should be centrifuged again.
Transferring and Drying the Methylene Chloride Solution
1. Using the plastic pipette, remove the lower organic layer and transfer it to a 50 mL Erlenmeyer flask, try not to
transfer any of the dark aqueous layer.
2. Add a fresh 3 mL portion of methylene chloride to each tube, cap and shake for about 30 seconds to extract the tea
again. Be sure to occasionally vent the centrifuge tube.
3. Centrifuge the tubes again as described above.
4. Remove the bottom organic layer and combine with the first extracts in the 50 mL Erlenmeyer flask. Again, try
not to transfer any of the dark aqueous layer.
5. Add anhydrous sodium sulfate (Na2SO4) to the combined methylene chloride extracts to remove any traces of
water. Add small spatula tips full of the sodium sulfate until the crystals no longer clump. Allow this mixture to
stand for 10 minutes.
Evaporating the Solvent and Recording Mass of Caffeine
1. Weigh a second, dry 50 mL Erlenmeyer flask and record the mass on your data sheet.
2. Transfer the dried methylene chloride extracts to the flask using a dry pipette. Be careful not to transfer any of the
solids. Place the flask on the steam bath in the fume hood to evaporate the methylene chloride.
3. Remove the flask from the steam bath as soon as all the solvent has evaporated, otherwise you may lose some of
the caffeine by sublimation.
4. Dry the outside of the flask with a paper towel, weigh the flask with contents, and record the mass on your data
Measuring the Melting Point of the Recovered Caffeine (If time permits!)
1. Scrape as much of the caffeine from the flask as possible using a spatula.
2. Load a capillary tube with about 2 mm of the caffeine and determine the melting point using the Digi-Melt
3. Deposit the remaining caffeine in the container in the organic waste container under the hood.
1. Determine the mass of the tea leaves (before brewing) by taking the difference between the tea bag (with tea) and the
average mass of the empty paper bag itself (provided by the instructor).
2. Calculate the mass percent caffeine in the tea leaves.
3. Suppose your tea bag was used to brew a standard American “cup” of tea (which is actually 6 fluid ounces). Calculate
the concentration of the caffeine in the beverage in units of mg of caffeine per fl. oz.
Questions (Include in Your Lab Report Discussion)
1. Black tea often contains up to 5% caffeine by mass. How do your results compare with this value? What might
account for measuring a percentage lower than this value?
2. The literature melting point of caffeine is 238⁰C. How does your measure melting point compare? What does the
melting point measurement tell you about the purity of the recovered caffeine? (Remember, a depressed and
broad-ranged melting point is indicative of an impure sample.)