Rosie Steinbach

Ms. Sica

Chemistry 1-4

3 October 2010

Penny Density Lab

Up until sometime in the late twentieth century, the U.S. penny was about 95% copper. Because the value of copper in a penny became greater than the monetary value of one cent, the government began to make pennies that were 97.5% zinc. The purpose of this lab is to identify the exact year that the composition of a U.S. penny was changed from mostly copper to mostly zinc. If the density of a penny shifts from roughly 9 g/cm3 to about 7.5 g/cm3 in successive years, then the composition of pennies was changed in the second year.

Materials: Pennies from each year (1960-2010) Ruler Calculator Balance Paper Pencil

Procedure: 1) Measure the diameter and height of a penny (cm)2) Calculate volume using πr2∙height and record this value in a data table for all

pennies tested3) Using a balance, measure the mass in grams of a penny from one end of the

sample (either mid-nineties or twenty-first century) and record it in the data table4) Calculate density (m/v) and record it in the data table in g/cm3

5) Measure the mass and calculate the density of a penny from the other end of the sample and record these values in the data table

6) Repeat steps 3 & 4 for pennies of increasingly closer years7) By analyzing densities from each year, identify the year that the composition of

pennies was changed8) Write a conclusion in which you analyze percent error and address your

hypothesis and the purpose of the lab

Steinbach 1

Data Table: The Density of A U.S. Penny Over TimeYear 1981 1982 1983 1985 1991 1998 2001

Volume(cm3)

.34 .34 .34 .34 .34 .34 .34

Mass(g)

3.09 3.05 2.53 2.56 2.51 2.49 2.50

Density(g/cm3)

9.1 9.0 7.4 7.5 7.4 7.3 7.3

My hypothesis was correct in leading us to the conclusion that the density of the U.S. penny was changed between 1982 and 1983. In calculating density, we had an average of 2.0% error in pre-change pennies and 2.6% error in post-change pennies. Most likely, the majority of this error would have resulted from inaccurate measurements of the pennies’ width and diameter (this measurement was more subjective to human interpretation via a ruler vs. mass taken with a calibrated balance). Width especially was a very hard measurement to take accurately since its value was so small. The difference in percent error between pre- and post-change pennies probably arose because we tested almost twice as many pennies from the years after 1982, so there were more opportunities for inconsistency. However, overall, our measurements for density were fairly precise and, though definitely not perfect, reasonably accurate.

Steinbach 2