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