How to Write Lab Reports

Common Errors

Several hundred Chem 201/202 lab reports get read every year, and it probably won’t surprise you to learn that errors fall into distinct patterns. Here is a list of common errors and some recommendations for avoiding them.

Recommendations

Be explicit. A common error is to assume that the reader will take the lab manual as a given and only a few extra details need to be added (this is also a common error in lab notebooks). You should assume that your reader is an expert organic chemist who is, nonetheless, completely unfamiliar with the instructions in your lab manual. Your Results section must provide some description of what you did (and this should precede your observations and conclusions), and your Experimental must contain all of the details needed to repeat your experiment.
Follow a natural train of logic when describing your experiments. Your Results section tells a story. While it is helpful to have a ‘big picture’ (where is this story going?) in mind, start telling your story from the beginning, not the end. This means describing your experiments first, following this with the data and observations they generated, and then finally drawing conclusions. The reverse order doesn’t make sense. For example, “A pure product was obtained in good yield. It consisted of white crystals of salicylic acid. The acid came from methyl salicylate which had been boiled with aqueous alkali.” is very hard to follow. Much better is, “Salicylic acid was obtained by boiling methyl salicylate with aqueous alkali. The product was isolated as white crystals, mp 158-9 ºC (lit.¹ 158.6 ºC), in 73% yield.“
Build your arguments on thoughtful comparisons. You have described your experiment and drawn some conclusions. The conclusions reached in Chem 201/202 lab reports are quite modest compared to those found in a typical research article, but the same set of questions need to be addressed every time a scientist draws conclusions no matter how modest the outcome: Can the conclusions be accepted at face value? Do the data and observations point to only one conclusion or are other conclusions possible? Have other scientists performed related experiments? Are these results consistent with yours? Some comparison of your results with those of other scientists, even if it just a comparison of your chemical shifts or boiling point with literature values, is almost always necessary.
Use a spellchecker. Misspelled terms, especially scientific ones, can lead to a great deal of misunderstanding. Note: we make every effort to spell technical terms correctly in this manual. If you have questions about spelling, search for examples of the word in this manual.

Some (Very) Common errors

The ambiguous “it”. Example: “Ethanol was mixed with mineral oil, and it was recovered by vacuum filtration.” What does “it” refer to? Ethanol? Mineral oil? The mixture?
Starting sentences with numerals.
Inventing imaginary technical terms. “The solution was filtered” is correct, but “the solution was filtrated” is not. “Salt was weighed” is acceptable, but “salt was massed” is not.
Treating units as nouns. “I poured some mL of water into a beaker” is wrong. It should be “I poured a small amount of water into a beaker.”
Using units as labels in graphs and tables. Incorrect label seen on graph axis: “mL distillate.” Correct label: “Distillate volume (mL)”.
Using long, imprecise expressions instead of succinct technical terms. For example, use “distillate” instead of “the liquid that condensed from the vapor at the top of the column.” Likewise, suppose you had to refer to the two protons next to the carbonyl group of PhC(=O)CH₂CH₃. You could say “alpha-methylene protons” instead of “the protons on the carbon bonded to the carbon double bonded to oxygen.”
Run on sentences and paragraphs. A sentence describes a single thought and must satisfy the rules of grammar. A paragraph addresses a single topic and often contains a topic sentence.
Connecting independent data points on a graph.
Leaving some NMR signals out of the Experimental, or leaving out required data elements for these signals. All NMR peaks created by your compound must be listed and all required data elements must be included (see above). If you have questions about what is required, please see your instructor.
Rounding off too early in a calculation. You should carry insignificant figures through all of your calculations, and round off at the end. If you round off too soon, you may end up with a significant error.
Calculating % theoretical yield incorrectly. Yields are based on moles, not grams.
Calculating #moles reagent incorrectly. Use molecular weights to convert mass to #moles. Use density to convert volume to mass.
Reporting impossibly precise measurements. For example, our graduated cylinders have markings for every 0.1 mL, but it is common for students to report using “2.73 mL” of liquid because that is the amount that they had calculated as needed before the lab. Our graduated cylinders don’t permit this precision. The proper quantity to report is the amount that was measured in the lab. This could have been 2.7 mL or 2.8 mL, but never 2.73 mL.
Forgetting to substitute your observations and literature sources for the fake ones in the sample reports.