Chemistry 122, Spring, 1996


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A Penny for your Thoughts

1. What do you think is the chemical composition of pennies?

2. Have pennies changed over time? Inspect the collection of pennies provided by the instructor (you may augment our collection with some of your own pennies if you care to make a contribution). Consider the following hypotheses:

a. Perhaps, because of wear, the older pennies are lighter that the newer ones.

b. Possibility the pennies have changed in chemical composition over the years.

3. Devise an experiment to test the first hypothesis. You have access to an electronic scale that can measure the weight of a single penny to a precision of 0.01 gram. Devise and execute a data collection scheme and a graphical method of presenting your findings. Organize your group to divide the labor. Use as many pennies that you feel is necessary to come to some conclusion. If you need graph paper, ask for it.

4. Before beginning the measurements, make your own personal prediction as to what you will find:

5. Write up on the back of this sheet a description of the procedure that your group developed, the results that you obtained, and the conclusions your reached.

6. Where there any surprises? Speculate on the origin of any discrepancies that you observed.

7. Locate information on (a) the percentage composition of metallic elements in U. S. coins and (b) the constant physical properties of those elements that might be used to support compositional claims experimentally. I suggest using the World Wide Web (via Netscape, available in a WAM labs), but if you have convenient access to paper sources for the same information , you are free to use those. Refer to the experiment "Chemical Informatics" for instructions on how to perform keyword searches on the World Wide Web. Try such keywords as "United States", "Mint", " coin", "elemental" and "composition". (Coins are produced by the U. S. Mint). Ask the instructor for help if you have trouble.

8. How did that information help explain the laboratory observations you made in the last experiment?

9. Why would differences in chemical composition of coins produce differences in mass? In other words, what property of the metals that make up coins might explain why the mass of modern pennies (after 1983) would be less that older pennies, when their sizes (volumes) seem to have always been the same?

10. Assume, for simplicity, that new pennies are 100% zinc (actually they are 97.5% zinc, which is pretty close) and that old pennies are 100% copper. Write down a calculation that shows whether or not the differences in the densities of copper and zinc are enough to explain quantitatively the differences between the masses of old and new pennies?

11. You have no doubt worked with density and its measurement in previous schooling. Using common laboratory equipment, how could you measure the density of a coin experimentally? Of any irregularly shaped object?

12. Measure the average density of (a) old pennies, (b) new pennies, and (c) pure zinc, as accurately as possible, using the equipment in the lab. Report your measurements and calculations below.

13. Which measurement, mass or volume, is most difficult to make precisely and accurately? Why?

14. When measuring density, does it make any difference how accurately the volume is measured, as long as the mass is measured accurately enough? Explain.

15. If you express density in grams/cm3 (1 cm3 = 1 mL), estimate to what accuracy you can measure the density of metal in the way that you are doing it. (Hint: estimate how accurately you can measure mass, then how accurately you can measure volume, then consider what would happen to the density calculation if both the mass and the volume were in error).

16. Describe some ways of doing the experiment that would increase the accuracy of the measurement of the average density of coins.

17. Challenge question: Most modern coins are made of two metals layered together. Derive a general equation for calculating the overall density D of a coin that is made from any two metals A and B, with densities DA and DB , and that contains X % of A (and therefore 100-X % of B), by mass. Show your work on the back. [Worked solution to this problem]