Chemistry Department, University of Southern Maine

Guidelines for Reports, Abstracts, and Graphs

Some Good Advice

Prepare your lab report as soon as possible after the lab meeting so that you can remember what you did and how you did it. Working early will also give you time to ask questions if you have problems. If you complete your lab work and clean up your bench and equipment before the end of the period, use the remaining time to begin your calculations. Your instructor will be available to help you get off to a good start.

Reports

  1. Read each Report Form carefully to find out exactly what to hand in for each report.
  2. Hand in exactly what is called for, no more, no less. -- show your instructor that you can follow instructions precisely.
  3. Staple the parts together in the order requested. Do not include cover sheets or binders.
    NOTE: We accept only printed reports, not email attachments or faxes.
  4. If the Report Form calls for an abstract, a graph, or an estimate of experimental variation, consult the guidelines and examples below.

Abstract

An abstract is a brief description of the goals, methods, and results of a lab experiment. Your instructor will ask you to write abstracts for at least two experiments. Follow these guidelines:

  1. Prepare the abstract with a word-processing program.
  2. Write in the first person, active voice, past tense, unless your instructor specifies otherwise.
  3. Make the abstract an example of your best writing: organized, logical, and free of errors in grammar and spelling.
  4. Give the abstract an informative title.
  5. Begin the abstract with a topic sentence, in which you state the goal of the experiment.
  6. State the goal as a scientific accomplishment, not an educational accomplishment. For example, state that you carried out a series of reactions and determined the purity and yield of the final product (a scientific goal). Do not state that you ran some reactions in order to learn how to use basic lab equipment (an educational goal). Tip: Derive your goal statment from the goal stated for the experiment in the Online Manual.
  7. Briefly describe the method(s) you used to accomplish your goal.
  8. If you used or studied a chemical reaction, give the equation. Use subscripts and superscripts properly.
  9. If you used or studied a compound, give its formula. Use subscripts and superscripts properly.
  10. Do not give details of procedures or specific intermediate results.
  11. After describing methods, present the most important result(s) of the experiment. For example, if you were trying to measure the molarity of acid in a sample of vinegar, give your final computed molarity.
    ==> Failure to present your final results is the most serious content error you can make in an abstract.
  12. Finally, give a measure of the precision or accuracy of your result, whichever is appropriate:
    a) Describe precision by giving the amount of variation for several trials of your measurement, or
    b) Describe accuracy by comparing your value with a literature or expected value.
  13. Be brief. Maximum length is one-half page, single spaced.

Sample Abstract

Empirical Formula of a Compound

by Vivian Johnson

I determined the empirical formula of an oxide of bismuth by finding the ratio [(moles O)/(moles Bi)] in a sample of the compound. I heated three weighed samples of finely divided bismuth in air and obtained a yellow oxide product. By comparing the measured weights of the oxide products with the original weights of the bismuth samples, I found the percent by weight of Bi and O in the product. Then I calculated the molar ratio of O to Bi, obtaining a value of 2.46 ± 0.17 for the three trials. This ratio corresponds to an empirical formula of Bi2O5, which I assume to be the product of this reaction:

4 Bi (s) + 5 O2 (g) --> 2 Bi2O5 (s)

Bi2O5 corresponds to the formula of a known oxide of bismuth listed in the CRC Handbook of Chemistry and Physics.

 

Not that Vivian's stated goal is scientific, not educational. She does not say that she did this experiment to learn something about chemistry lab work. She says she did it to learn the formula os a bismuth oxide.

Note that Vivian's abstract includes no specific details of procedure, and no specific intermediate quantities, such as the masses of her three Bi samples or her three products. The only quantity that Vivian provides is the result that constitutes the goal of the experiment (the molar ratio of O to Bi), along with an assessment of its precision: the amount of variation in three trials. The abstract ends with a brief interpretation of the meaning of the results, and a comparison with literature values.

Note also that Vivian includes an equation for the reaction she ran in this experiment, as well as the formula of the most important substance under study. The equation and formula show that Vivian has figured out how to make subscripts in the word processor.

Now look again at the guidelines for writing abstracts (above) and see for yourself that this abstract meets all of the requirements.

 

Graphs

A graph should be able to stand alone as a description of a chemical system and its behavior.

Follow these guidelines for all graphs:

  1. Adjust the scale of your graph so that the plotted data covers most of the graph in both the x and y directions.
  2. If your graph will contain plotted data plus a fitted line or curve (called a "trendline" in Excel), plot a scatter graph without connecting lines, and make sure that the data points are displayed as separate points. The only line or curve on the graph should be the trendline.
  3. Label the axes with variable names, followed by abbreviations of units of the variable in parentheses: Example: Concentration of S2O82- (mol/L).
  4. Divide the axes into easy-to-understand units.
  5. Give an informative title that adds to, rather than repeats, the information on the axes. Example, for a graph of rate constant versus temperature: Effect of Temperature on Rate of Oxidation of Iodide Ion by Peroxydisulfate. The title should not simply reiterate the names of the variables; they are obvious to the reader if the axes are properly labeled.
  6. Display the equation of a line or curve fitted to the data (if applicable). Edit the equation to change the default variable names (usually x and y) to the actual names of the variables you graphed.
  7. Avoid scientific notation on graph axes.
    Scientific notation in on Excel graphs is very unprofessional in appearance.
    Here are two ways to avoid it:
    • Change data format in cells. If scientific notation is not needed (plotted data in the range of 0.01 to 1000, use the Excel command Format:Cells... to set the format to Number, and specify a number of decimal places that makes for a neat display on your graph axes. This action will prevent the numbers of graph axes from having needless repeated notations like "E+00".
    • Rescale data. If axis numbers are smaller than 0.001 or larger than 1000 (the situation in which you usually use scientific notation), avoid scientific notation on axes by multiplying or dividing all numbers by a constant factor that results in rescaled data values between 0 and 10, or 0 and 100. Then label the axis to show how to interpret the axis numbers. For example, if you multiply a set of small mass values in g by 105 and plot them, then label the mass axis as follows: Mass (g) x 105. This means that a value of 10 on that axis should be interpreted as "10 = mass x 105", which means that mass = 10/105 = 1 x 10-4. Even after doing this you may still need to use the Format:Cells... command to get rid of those ugly "E+00" labels.

Sample Graph

Now look again at the guidelines for graphs (above) and see for yourself that this graph meets all of the requirements.

Estimating Experimental Variation

(Sometimes Called Experimental Error)

This appendix applies specifically to the CHY 116 experiment "Determining an Equilibrium Constant". You can also use the reasoning presented here to assess the reliability or precision of any laboratory results.

Experimental variation is a measure of how reliable your lab results are, assuming that you carried out the procedure properly, and made all measurements with proper technique and care. It is sometimes called experimental error, but that term is misleading, because experimental variation tells you how precise your results are if you made no errors. Experimental variation depends only on the precision of your measuring tools.

In the experiment on Kc for FeSCN2+, you used 5-mL graduated pipets to measure volumes, and you can read these pipets to a tolerance of about ± 0.02 mL. This tolerance would introduce a maximum error of 2% in a 1-mL volume, the smallest volume you measured (solution E2). Because of drift in the last decimal place on the Spec-20, you can read A to a tolerance of about ± .005, introducing a maximum error of 0.005/0.20, or 2.5%, in your smallest measured absorbance (about 0.20, also in solution E2).

After you calculate Kc, if you calculate it again, but you increase [FeSCN2+] by 2.5% (because you determine it from A) and decrease [Fe3+] and [SCN-] by 2% (because these molarities depend mostly on volume measurements), this will compound the errors in the worst way possible, and give a value of Kc that contains the maximum expected error or variation. Try it for one of your calculated values of Kc.

Sample Calculations

Kc = [FeSCN2+] / [Fe3+][SCN-]

Without error: Kc = (3.84 x 10-5 ) / (9.62 x 10-4 )(1.61 x 10-4 ) = 248.

Adding 2.5% to the numerator, and subtracting 2% from each molarity in the denominator gives this result:

With error: K'c = (3.94 x 10-5) / (9.43 x 10-4)(1.58 x 10-4) = 264.

The difference between Kc and K'c is 16, which is about 6.5% of 248. The maximum expected experimental variation in Kc is therefore 6.5%.

This example shows how to use the precision of lab instruments to estimate the expected variation in results. This method gives the maximum error you can expect in Kc if you make no blunders in lab.

CHY 114 Syllabus

CHY 116 Syllabus

Lab Manuals