Honors
Chemistry
Lab
Fall 2001
Isotopic Pennies
Introduction:
Materials:


Pennies
Scale
Most elements on the periodic table have more than one form with different
masses. These different versions of the elements are known as isotopes.
Isotopes have the same number of protons, but a different number of
neutrons which gives them different atomic masses. When you have a sample
of an element, it contains all of the forms of that element in a fixed ratio.
In this lab, we are going to take advantage of the change in the mass of
pennies over the years to mimic isotopes of a particular element. In 1982, the
U.S. Mint began changing the composition of the penny and consequently its
mass. Before 1982 the penny was primarily a copper alloy (mixture of metals).
After 1982 the penny became primarily zinc with a copper coating.
Procedure:
1. Prepare a data table similar to the
one on the following pages in
your lab notebook. All data
should be recorded in your lab
notebook.
2. The pre-1982 and post 1982 pennies have all been mixed together. Record the total number of
pennies in the container, the
number of pre-1982 pennies, and
the number of post-1982 pennies. Generate a % composition
for the container.
3. Find the mass of 10 pre-1982
pennies and 10 post-1982 pennies. Calculate the average mass
of both pre and post-1982 pennies.
4. Predict the mass of a 5-penny
sample using the weighted average mass of a random penny that
you just calculated and record
this number in your table.
5. Remove a random sample of 5
pennies and record their mass.
5. Predict the mass of a 10-penny
sample using the weighted average mass of a random penny and
record this number in your table.
6. Remove a random sample of 10
pennies and record their mass.
7. Predict the mass of a 100-penny
sample using the weighted average mass of a random penny and
record this number in your table.
8. Remove a random sample of 100
pennies and record their mass.
Page 2
Name:
Total # of pennies in
container
Isotopic Pennies
How are the pennies you used in this lab like atoms?
# of pre-1982 pennies in
container
# of post-1982 pennies in
container
% pre-1982 pennies
% post-1982 pennies
How are they different?
Mass of 10 pre-1982
pennies
Average mass of pre-1982
penny
Mass of 10 post-1982
pennies
Average mass of post-1982
penny
Weighted average mass of
random penny
Calculate the % error for your predicted 5 penny mass
sample, your 10 penny mass sample, and your 100 penny mass sample. Show all work.
Predicted mass of 5 penny
sample
Actual mass of 5 penny
sample
Predicted mass of 10 penny
sample
Actual mass of 10 penny
sample
Predicted mass of 100
penny sample
Actual mass of 100 penny
sample
Based on the trends in your data, do you think this will be an accurate way to predict the mass of a sample
of an element? Why or why not?
Although isotopes of an element generally behave the same chemically, certain ones have unique properties
that can be very useful. Look up the following isotopes in your textbook and write their uses below.
Co-60
U-235
C-14
Tc-99.