Tips for Lab Reports
CHY 114 & 116
CHY 114 & 116
Chemistry Department
University of Southern Maine
Portland, Maine 04104-9300
Table of Contents
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:
- Include the origin (0,0) at the lower left of the graph, if practical.
- Label the axes with variable names, followed by abbreviations of units of the variable in parentheses: Example: Concentration of S2O82- (mol/L).
- Divide the axes into easy-to-understand units.
- 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.
- 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.
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 22). 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 22).
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. Here is an example:
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.
Sample Abstract (CHY 114 or 116)
A Summary in CHY 116 is just like an Abstract in CHY 114, except that you write the summary informally in your lab notebook, at the end of all your lab records for an experiment (procedure, observations, data, calculations, results conclusions).
Here is an additional example of an abstract:
Empirical Formula of a Compoundby Vivian Johnson
I determined the empirical formula of bismuth oxide 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. 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 is the formula of a known oxide of bismuth listed in the CRC Handbook of Chemistry and Physics.
Note that the abstract includes no specific details of procedure, and no specific intermediate quantities. The only quantities that Vivian provides are the results that constitute the goal of the experiment (the molar ratio of O to Bi), along with an estimate of its precision. The abstract ends with a brief interpretation of the meaning of the results, and a comparison with known values.
Now look again at the guidelines for writing abstracts (in your lab manual) and see for yourself that this abstract meets all of the requirements.