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Subject: Spreadsheet Calculations Of %C In Aliphatic Compounds

High school chemistry teachers that teach an introductory organic unit and also expect students to use computers to do calculations in spreadsheets may want to try this activity. A template on a 8.5" x 11" sheet of paper is provided to each student. The construction and use of the template follow.

1. The column headings are as follows;
Alkane name number of C atoms number of H atoms molar mass %C %H
2. The same headings are repeated for the alkenes and the alkynes.
3. The students are instructed to type in the name of the first 10 alkanes, alkenes and alkynes in the appropriate columns.
4. A cell formula that calculates molar mass is given to the students. They are instructed to type it into the methane row of the chart and then to "fill-down".
molar mass = #C*12.011 + #H*1.0079
5. The percent of carbon by mass for each formula is calculated from %C = #C*12.011/molar mass
%H is calculated the same way.
(Let the computer multiply by 100 when you convert the number format to percent.)
6. Repeat these calculations in the other sections of the chart reserved for the alkenes and the alkynes.
7. This completes the spreadsheet but it needs analysis. I recommend Vernier's Graphical Analysis program for plotting %C vs. the number of carbons in the series.
8. Use 3 data series on the graph to compare the effects of increasing carbons on the percent of carbon. The results may be surprising to many students and teachers as well. (I was surprised.)
9. The alkanes increase their percentage of carbon until an assymtotic value of about 85% is achieved at many 100's of C. The alkynes decrease their percentage of C until they also approach their asymptotic value of about 85%. The alkenes with a constant C/H ratio have a constant percent of about 85% Carbon. (no surprise here)
10. Have the students interpret the graph in their own words but suggest that they use the word "asymptote" in their discussion.
11. Have fun with this. I did.

Larry Lemmert (llemmert@dataex.com)
Neenah High School, Neenah, Wisconsin

I. The formulas shown above are for ClarisWorks and may need to be modified for another spreadsheet program.
II. Vernier Software, 8565 SW Beaverton-Hillsdale Hwy, Portland, OR 97225-2429.
Ph. (503) 297-5317
Fax (503) 297-1760