CP Chemistry
Theodore Roosevelt High School
Lab #2-4
Composition of a Penny Lab
Introduction
As you know, the United States one-cent coin, or the penny, is a unit of
currency equaling one one-hundredth of a United States dollar. However,
you may not know that the composition of the penny has changed
dramatically over the years. In 1943, at the peak of World War II, cents of
zinc-coated steel were made for a short time due to war demands for
copper. During the early 1970s, the price of copper rose to a point where
the cent almost contained more than one cent's worth of copper. This led
the Mint to test alternate metals, including aluminum and bronze-clad
steel, though neither were adopted. Because the value of the copper in the coin eventually rose above
one cent, the cent's composition was finally changed in 1982 to its current make-up of an inside zinc
core surrounded by a thin copper coating.
In this lab, we will determine the percent composition of a modern (post-1982) penny by using a strong
acid to react and dissolve the zinc core, leaving only the copper coating. Once only copper remains,
we will compare its mass to the entire mass of the penny to determine how much of a penny is copper
and how much is zinc.
Purpose
To practice calculating percent composition.
Prediction
What percentage of your penny do you think will be made of zinc? Why do you think so?
Equipment
beaker, 100 mL
metal file
oven
Materials
hydrochloric acid, 1M [HCl]
penny (1983 or later)
Procedure
Day 1:
1.
Obtain a clean post-1982 penny.
2.
Measure and record the mass of the penny in your lab notebook.
3.
Using a metal file, file three, oppositely-placed small grooves into the edge of the penny. The
grooves must be deep enough so that the zinc is exposed, but not so deep that the penny's mass
is greatly affected.
4.
Obtain a small beaker and label it with your name, your partner’s name and your period.
5.
While wearing gloves, carefully pour 75 mL of 1M HCl into a 100-mL beaker.
6.
Carefully place the penny in the acid. Observe the effect the acid has on the copper outside of
the penny.
7.
Place your labeled beaker under either fume hood to react overnight.
CP Chemistry
Theodore Roosevelt High School
Lab #2-4
Day 2:
1.
After the penny has reacted overnight, carefully pour some water from another beaker into the
acid to dilute it.
2.
Carefully remove the remainder of the penny with a pair of tongs.
3.
Gently rinse the penny with water from a wash bottle and pat it dry with paper towels.
4.
If any zinc inside the penny remains unreacted, repeat steps #5-7 from Day 1 and check again the
next day.
5.
Put the penny in a small beaker and place it in the oven for 5-10 minutes to allow it to dry
completely.
6.
Dispose of the acid in the large waste beaker in the fume hood (DO NOT POUR IT DOWN THE
SINK).
7.
Remove the penny and measure and record its new mass in your lab notebook.
8.
Dispose of the penny in the trash and clean all used glassware.
Data – you should create a data table in your lab write-up that looks something like this:
Day 1 Mass
Day 2 Mass
Mass of Zinc*
% Composition*
% Error*
Calculations
1.
Calculate the mass of zinc in the penny that was reacted.
2.
Knowing the total mass of the penny and the mass of both copper and zinc in the penny, calculate
the percent composition of your penny.
3.
According to the United States Mint, pennies are 97.5% zinc and 2.5% copper. Calculate the
percent error for your experimental percent composition of zinc.
4.
Using the accepted density of zinc (7.140 g/cm3) and the accepted volume of a penny
(0.360 cm3), calculate what the mass of a penny would be if it were made of solid zinc.
Questions
1.
Why is the mass of your penny so close to the mass of the hypothetical “solid zinc penny”
calculated in Calculation #4?
2.
Copper and zinc are next to each other on the period table. How is it possible that one of them
would react with hydrochloric acid, but not the other?
3.
Why is it necessary to use a post-1982 penny for this lab?
Errors
Describe two possible errors you may have committed in this lab that may have somehow affected your
results. Explain the specific steps you will take to avoid each of these errors in the future.
Conclusion
Write two or more paragraphs summarizing your results, examining the validity of your prediction, and
describing what you learned while doing this lab.
CP Chemistry
Theodore Roosevelt High School
Composition of a Penny Lab workspace:
Lab #2-4