Molecular Weights by Experiment
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Molecular Weights by Experiment

You have seen how the formula weight for a chemical can be determined from the formula for that chemical. Do you think that the formula weight can be determined if the formula is not known? The answer is yes, or perhaps I should say the answer is sometimes. That is what you will do in the experiment for this lesson. You will determine the formula weights (specifically, molecular weights) of several gases.

This experiment is intended to challenge you to figure out how to use the data you collect to determine the molecular weights of several gases. The guiding principle that you will use is Avogadro's Law which we talked about earlier in the lesson. Equal volumes of gases, measured under the same conditions, contain equal numbers of molecules. Thus, if two equal-volume gas samples are weighed under the same conditions and the first sample weighs 2.53 times as much as the second, then the molecules of the first gas must weigh 2.53 times as much as the molecules of the second gas. If the molecular weight of one gas is known, the molecular weight of the other gas can be calculated from it, but you need to determine how to do those calculations. Think about how you plan to attack this problem and talk it over with other students.

Expectations for the Lab Report

You will need to turn in a lab report on this experiment in the style that you learned previously. It should be typed, although you may write in the calculations by hand if it is too cumbersome to figure out how to type them in an understandable format.  Be sure to include your name and the names of any coworkers and a title. Then state the purpose of the experiment. List and label the data you obtained in your measurements. That can be similar to the table in your workbook or in your own style, but be sure to label the data properly. Then show how you calculated the formula weights of the gases. This is particularly important in this experiment because the instructions do not tell you exactly how to do this. So you must make clear in your report what you have done. Be sure to take significant digits into account. Then in the results section report the formula weights of the gases that you determined in this experiment. If you have comments to make about problems you ran into or variations that you made in the procedure, include those in a comments section. After that, write out your answers to the questions about the experiment that are listed in the workbook in example 15e. Your report for this experiment will be graded on the accuracy of your data and results, the correctness of your calculations, the answers to the questions, the format of your report, and even the neatness of your report.

The nature of this experiment is such that you can either do it now if you are in the lab or later in the lesson, whichever fits your schedule best.


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E-mail instructor: Sue Eggling

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