AP Chemistry Lab #1
1
Gravimetric Analysis of the Percent Composition of a Copper Penny
PURPOSE
To determine the percentages of copper and zinc in a penny.
BACKGROUND
Pennies dated up to 1981 are made of copper. Post 1983 pennies are not all copper. Instead they have a zinc core
that is surrounded by copper plating. By chemically removing the zinc from the center of a pre-weighed penny, the
percentage of copper and zinc can be determined. The U. S. Mint specifies pennies as 2.5% copper and 97.5% zinc
(by mass).
DEFINITIONS
Mass percent, mole fraction, single replacement reaction.
PROCEDURE
Day 1
1. Label a 50mL or 100mL beaker with your name.
2. Obtain a penny dated 1983 or later. Record the date of your penny.
3. Using a triangular file make two scratches on the surface of the top and bottom portions. For best results
make the scratches 180° apart (see diagram). The zinc core should be visible.
Filed topside area
Filed underside area
4. Clean the top and bottom surfaces with steel wool until shiny. Don’t be too abrasive. Clean and dry the
penny thoroughly and determine its mass on the balance. Record the mass of the penny.
5. Remove the penny from the balance with tweezers and place in the beaker . Add about 25 mL of 4 M HCl
to the beaker.
6. Record observations about the system. Place the beakers in the fume hood until class the next day.
Day 2
7.
Record observations before removing the penny from the hydrochloric acid. CAREFULLY remove the
penny from the beaker, and “feel” with the tweezers to insure that the zinc core is entirely removed. If it is
not, soak in 2 M HCl overnight again. Empty the used HCl solution in the beaker down the sink with the
water running.
8.
If you determined that all the zinc has been removed, carefully rinse the Cu skin several times in fresh
distilled water. Again, empty the used HCl solution in the beaker down the sink with the water running.
9.
Rinse out the beaker several times and fill half full with distilled water.
10. Place the penny back into the beaker and allow to soak in the distilled water overnight.
Day 3
11. Using tweezers, remove the copper skin and immerse in an acetone rinse for about 2 minutes. Acetone
will remove any water and evaporate much faster than water.
12. Remove the copper skin from the acetone bath and place on a paper towel. Let dry 10 minutes.
13. Weigh the copper skin and record the mass.
ANALYSIS
1. Write a balanced chemical equation for the reaction of hydrochloric acid and zinc.
2. Determine the percent by mass of zinc and percent by mass of copper in the penny.
3. Determine the mole fraction of zinc and mole fraction of copper in the penny.
EXPERIMENTAL ERROR
If the zinc core had impurities that did not react or dissolve in HCl(aq), how would the percent copper in the penny
be affected?