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Chemistry I
Name ______________________________________
Title: The Density of Pennies
Background: The original Lincoln penny was made of pure copper. In 1982 the
composition of the penny was changed due to the cost of copper metal. Currently, the
penny is manufactured by covering a zinc core with copper metal, followed by stamping
the front and back images in a mold. The density, eq. 1, of an object can provide
information concerning its composition, since every pure substance has a unique density
and mixtures result in densities that represent the weighted average of its composition.
Density =
Mass
Volume
(1)
In this experiment you will determine the density of pennies before and after 1982. Since
the pennies minted before 1982 are known to be essentially pure copper, the results
obtained will provide an estimate of the accuracy of the experiment and act as a control.
For the pennies that were minted after 1982, the density should provide an estimate of the
percentages of copper and zinc in the modern penny.
Materials: A large number of U.S. pennies, a triple beam balance, 50 mL graduated
cylinder, water, graph paper and pencil.
Procedure: Sort your pennies into two groups, pennies minted before and after 1982. Do
not use pennies minted in 1982, because they may consist of both types of pennies. Place
approximately 20 mL of water in a graduated cylinder and record the volume as
accurately as possible in Table 1. Weigh and record the mass, as accurately as possible,
of a few dry pennies minted before 1982. Carefully place these pennies in the graduated
cylinder to avoid splashing and remove any bubbles that may be present by gently
shaking the cylinder. Record the new volume reading, as accurately as possible, in Table
1. Weigh an additional group of pennies and add them to the graduated cylinder,
recording the appropriate total mass of pennies and volume data in Table 1. Obtain a
total of five data points by adding an additional three groups of pennies to the cylinder.
Your partner should follow this process to obtain data for the pennies minted after 1982.
Finally, determine the mass of two individual pennies minted before and after 1982 and
record this data in Table 1.
Using the graph paper, plot both sets of mass (y-axis) and volume (x-axis) data on the
same graph. Be sure to label the axes and include units. Using different colored pencils,
draw best-fit lines for both sets of data. Determine the slope of both best fit lines and
record this data in Table 1. The known densities of copper and zinc are 8.96 g/mL and
7.13 g/mL, respectively. Calculate a percent error, eq. 2, for the pennies minted before
1982 and record this data in Table 1. Assuming that the pennies minted after 1982 are
mostly zinc, calculate a percent error and also record this value in Table 1.
%Error = [(known – experimental)/known] x 100
© 2005 Van Der Sluys
(2)
Chemistry I
Name ______________________________________
Table 1. Data for the densities of pennies minted before and after 1982.
Pennies minted before 1982
Total No.
Pennies
Total Mass (g)
0
0.000
Volume (mL)
Pennies minted after 1982
Total Mass (g)
Volume (mL)
0.000
Slope
(g/mL)
Density
(g/mL)
% Error
(eq, 2)
Individual
Mass (g)
Data Analysis
1. What are possible sources of error in this experiment?
2. Divide the total mass of pennies for the final trials of pennies before and after
1982, by the total number of pennies, to obtain an average mass of the pennies.
Compare the average to the mass of the individual pennies and comment on the
agreement between these sets of data?
3. Using the density equation and the densities measured in this experiment,
calculate the volume of the two individual pennies from their measured masses.
Are the volumes similar? Why are the masses different?
© 2005 Van Der Sluys