Detailed Example of a Chemistry Lab Report

A chemistry lab report is an essential part of scientific learning, as it allows students and researchers to record, analyze, and present experimental findings in a clear and organized way. Writing such a report not only demonstrates an understanding of the scientific method but also helps develop communication skills crucial for academic and professional success. A typical chemistry lab report includes key sections such as the title, abstract, introduction, materials and methods, results, discussion, and conclusion. Each section serves a specific purpose, from outlining the experiment’s goal to interpreting the data collected. By following this structured format, the report provides transparency, making it possible for others to understand, replicate, or evaluate the experiment.

Sections of a Chemistry Lab Report

1. Title

• Purpose: To concisely state the subject of the experiment.
• What to Include:
  • Be specific and descriptive. Avoid generic titles like “Lab Report 1.”
  • It should often include the key reaction, technique, or compound studied.
  • Good Example: “Synthesis and Spectroscopic Analysis of Aspirin.”
  • Bad Example: “Chemistry Lab.”

2. Abstract

• Purpose: A brief (usually 100-200 words) summary of the entire report. It is often written last but appears first.
• What to Include: Answer these questions in a single paragraph:
  • Objective: What was the main goal of the experiment?
  • Key Methods: What primary technique or procedure was used? (e.g., titration, distillation, spectroscopy)
  • Key Results: What was the most important finding? Include quantitative data (e.g., yield, concentration).
  • Conclusion: What is the significance of your results? Did you achieve your objective?

3. Introduction

• Purpose: To provide the background context and theory needed to understand the experiment.
• What to Include:
  • Background: Explain the chemical principles, reactions, or theories relevant to the experiment (e.g., the principle behind acid-base titration, the mechanism of the synthesis reaction).
  • Objective: Clearly state the specific aim of the experiment. What question are you trying to answer?
  • Hypothesis: If applicable, state what you expect to happen and why.

4. Experimental Section (Materials and Methods)

• Purpose: To describe how you performed the experiment in enough detail that a skilled chemist could replicate your work exactly.
• What to Include:
  • Materials: List chemicals used (with purities and concentrations) and equipment.
  • Procedure: Describe the steps you took in the first person past tense (e.g., “We dissolved 2.1 g of solid X in 50 mL of water.”). Do not copy the lab manual verbatim; paraphrase it. Note any deviations from the prescribed procedure.
  • Safety: Mention any specific safety precautions taken (e.g., “Reaction was carried out in a fume hood due to the evolution of toxic gas.”).
  • This section should be factual and descriptive, not explanatory.

5. Results

• Purpose: To present the data and observations you collected without interpreting them.
• What to Include:
  • Qualitative Data: Record all observations (color changes, formation of a precipitate, gas bubbles, etc.).
  • Quantitative Data: Present all numerical data clearly.
    • Tables: Use tables to organize raw and processed data (e.g., masses, volumes, titration readings, calculated results).
    • Calculations: Show one sample calculation for each type of computation. For example, show the full equation and steps for calculating percent yield.
    • Figures: Include graphs, spectra, or chromatograms. All figures must have a caption (e.g., “Figure 1: Calibration curve for absorbance vs. concentration of copper(II) sulfate.”).

6. Discussion

• Purpose: This is the most important section. Here, you interpret your results and explain their significance.
• What to Include:
  • Interpretation: Explain what your results mean. Do they support the theory? Did the reaction work as expected?
  • Analysis of Data: Compare your final result (e.g., percent yield, melting point, concentration) to expected or theoretical values.
  • Error Analysis: Discuss the accuracy and precision of your results. If there are discrepancies, identify potential sources of error. Be specific (e.g., “A low percent yield (65%) may be due to loss of product during filtration,” not just “human error”).
  • Answer the “Why”: Why did you observe what you observed? Connect your reasoning back to chemical principles.

7. Conclusion

• Purpose: To briefly summarize the findings and their implications.
• What to Include:
  • Restate the main objective of the experiment.
  • Briefly state the key result(s).
  • State whether the objective was achieved and what you learned overall.
  • This should be a short, concise paragraph. The Discussion is for detailed analysis; the Conclusion is for the final take-home message.

8. References

• Purpose: To cite any external sources of information you used.
• What to Include:
  • List all sources, such as the lab manual, textbook, journal articles, or online databases (like CRC Handbook or PubChem).
  • Use a consistent citation style (e.g., APA, MLA, ACS style).

9. Appendices (Optional)

• Purpose: To include supplementary material that is too bulky for the main body of the report.
• What to Include:
  • Pages of extensive raw data.
  • Detailed, complex calculations.
  • Copies of instrument printouts (like NMR or IR spectra) if they were not included in the Results section.

Example Chemistry Lab Report

Title

Synthesis, Purification, and Spectroscopic Analysis of Acetylsalicylic Acid

Abstract

This experiment aimed to synthesize acetylsalicylic acid (aspirin) from salicylic acid and acetic anhydride via a nucleophilic acyl substitution reaction, catalyzed by sulfuric acid. The crude product was purified using recrystallization. The success of the synthesis and purity of the product were assessed by measuring the melting point and obtaining an IR spectrum. The final mass of purified aspirin was 2.41 g, corresponding to a percent yield of 72.8%.

The measured melting point range of the product was 135-137 °C, which compares well with the literature value of 135-136 °C. The IR spectrum showed a strong carbonyl stretch at 1750 cm⁻¹, characteristic of an ester, and the absence of the broad O-H stretch of salicylic acid between 2500-3300 cm⁻¹. These results confirm the successful synthesis of relatively pure acetylsalicylic acid.

1. Introduction

Acetylsalicylic acid, commonly known as aspirin, is one of the most widely used medications in the world, acting as an analgesic, antipyretic, and anti-inflammatory agent. It is synthesized through an esterification reaction, where the phenolic hydroxyl group of salicylic acid is acetylated.
The reaction involves salicylic acid reacting with acetic anhydride in the presence of an acid catalyst (sulfuric acid, H₂SO₄) to produce acetylsalicylic acid and acetic acid as a byproduct (Figure 1). This is an example of a nucleophilic acyl substitution.

Figure 1: Reaction Scheme

Salicylic Acid + Acetic Anhydride --(H₂SO₄)--> Acetylsalicylic Acid + Acetic Acid

The objective of this experiment was to synthesize aspirin and purify the crude product via recrystallization. The purity and identity of the final product were evaluated by calculating the percent yield, determining the melting point, and analyzing the functional groups present using infrared (IR) spectroscopy. A pure sample of aspirin should have a sharp melting point near 135-136 °C and an IR spectrum showing a characteristic ester carbonyl stretch and the absence of the broad alcohol O-H stretch present in the starting material, salicylic acid.

2. Experimental Section

Materials: Salicylic acid (2.00 g), acetic anhydride (5.0 mL), and concentrated sulfuric acid (5 drops) were used. Equipment included a 125 mL Erlenmeyer flask, hot plate, ice bath, Büchner funnel, and filter paper.

Procedure:
Salicylic acid (2.00 g) was placed into a 125 mL Erlenmeyer flask. Acetic anhydride (5.0 mL) was added, followed by 5 drops of concentrated sulfuric acid, which acted as a catalyst. The mixture was swirled gently until the salicylic acid dissolved. The flask was then heated in a hot water bath (~80 °C) for 10 minutes. After heating, the flask was removed and 2 mL of deionized water was slowly added to decompose any excess acetic anhydride.

The flask was then placed in an ice bath to crystallize the aspirin. Once crystallization was complete, the crude solid was collected by vacuum filtration using a Büchner funnel and washed with two small portions of cold water. The crude product was allowed to dry on the filter under vacuum for 5 minutes.

The crude aspirin was purified by recrystallization. The solid was dissolved in a minimum volume of hot ethanol (~4 mL) in a hot water bath. Deionized water (~10 mL) was added to the hot solution until it became cloudy, indicating saturation. The solution was then cooled slowly to room temperature and then in an ice bath to complete crystallization. The purified crystals were collected via vacuum filtration, washed with cold water, and allowed to dry thoroughly. The mass of the dry, purified product was recorded.

A small sample of the purified product was used to determine the melting point range using a Mel-Temp apparatus. Another small sample was prepared as a KBr pellet for analysis by Fourier Transform Infrared (FTIR) spectroscopy.

3. Results

3.1. Mass and Yield

• Mass of salicylic acid used: 2.00 g
• Mass of purified acetylsalicylic acid (ASA) obtained: 2.41 g
• Molar mass of salicylic acid (C₇H₆O₃): 138.12 g/mol
• Molar mass of acetylsalicylic acid (C₉H₈O₄): 180.16 g/mol

Theoretical Yield Calculation:
Moles of salicylic acid used = 2.00 g / 138.12 g/mol = 0.0145 mol
The reaction has a 1:1 stoichiometry, so the theoretical moles of ASA = 0.0145 mol.
Theoretical mass of ASA = 0.0145 mol × 180.16 g/mol = 2.61 g

Percent Yield Calculation:
Percent Yield = (Actual Yield / Theoretical Yield) × 100% = (2.41 g / 2.61 g) × 100% = 92.3%

3.2. Melting Point
The measured melting point range of the purified aspirin was 134-136 °C. The literature melting point for pure acetylsalicylic acid is 135-136 °C.

3.3. Infrared Spectroscopy
The IR spectrum of the purified product is shown in Figure 2. Key absorption peaks are identified in Table 1.

Figure 2: IR Spectrum of Purified Product
(A descriptive caption would be here, e.g., “FTIR spectrum of the synthesized product (KBr pellet).”)

Table 1: Key IR Absorptions

Wavenumber (cm⁻¹)	Bond / Functional Group	Vibration Mode
~1750	C=O (ester)	strong, sharp stretch
~1690	C=O (acid)	strong, sharp stretch
~1200-1300	C-O (ester & acid)	strong stretch

4. Discussion

The synthesis of acetylsalicylic acid was successful, as evidenced by the analytical data. A relatively high yield of 92.3% was obtained, indicating an efficient reaction and purification process. The high yield can be attributed to the use of an excess of acetic anhydride, which drove the equilibrium towards products, and an effective recrystallization that minimized product loss.

The melting point of the purified product (134-136 °C) was sharp and matched the literature value (135-136 °C) almost exactly. A sharp melting point that correlates well with the known value is a strong indicator of a pure compound. The absence of a depressed or broad melting point suggests that significant impurities, such as unreacted salicylic acid or acetic acid, were removed by the recrystallization.

The IR spectrum provides definitive evidence for the structure of the product. The presence of a strong carbonyl stretch at ~1750 cm⁻¹ is characteristic of an ester functional group, which is the key new bond formed in the reaction. The carbonyl stretch for the carboxylic acid group is also visible at ~1690 cm⁻¹, which is expected in the aspirin molecule. Critically, the spectrum does not show a very broad O-H stretch in the 2500-3300 cm⁻¹ region, which is typical of the carboxylic acid group in salicylic acid that is influenced by intramolecular hydrogen bonding. The absence of this broad peak confirms that the phenolic -OH group of salicylic acid was successfully acetylated.

Potential sources of error that led to the less-than-100% yield include minor losses of product during the transfer steps of recrystallization and the inherent solubility of the product in the mother liquor during vacuum filtration. There were no significant safety issues, as the reaction was performed with care, and the corrosive acetic anhydride and sulfuric acid were handled appropriately.

5. Conclusion

The objective of synthesizing and characterizing acetylsalicylic acid was achieved. From 2.00 g of salicylic acid, 2.41 g of purified aspirin was obtained, yielding 92.3%. The melting point (134-136 °C) and IR spectroscopic data confirmed the identity and high purity of the final product. The results demonstrate the effectiveness of Fischer esterification and recrystallization as techniques for the synthesis and purification of organic solids.

6. References

1. Pavia, D. L.; Lampman, G. M.; Kriz, G. S.; Engel, R. G. A Small-Scale Approach to Organic Laboratory Techniques, 4th ed.; Cengage Learning: Boston, 2015.
2. “Acetylsalicylic Acid.” PubChem, National Library of Medicine, pubchem.ncbi.nlm.nih.gov/compound/Aspirin. Accessed 18 Oct. 2023.

Appendix

Raw data notebook pages are attached.
Printed IR spectrum is attached.

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