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Block B
9/5/14
Penny Density Lab
Introduction
The density of an object can be found by dividing the mass of an object by its volume. While copper alone was used in pennies made before 1982, all pennies made since have been only copper plated and are made of a different metal on the inside. The aim of this lab was to determine the metal that constitutes the post-1982 penny. The slope of a mass vs volume graph represents the density of an object, so we used electronic balances to assess the masses of the pennies and water displacement to indicate the volume of space occupied by those pennies. We would then be able to compare the experimental density of the post-1982 to a table with the densities of several metals, in order to identify the unknown metal in the post-1982 pennies.
First, we filled up a graduated cylinder with 20 mL of water. We then assessed the mass of 5 pre-
1982 pennies and recorded the change in the volume when we put them in the water. We repeated this process two more times, each time adding five pennies to the cylinder. We then emptied the cylinder and did the exact same procedure with post-1982 pennies. We kept a chart to record our data and used our values to create a graph and find the density of both the pre-1982 and post-1982 pennies.
Data/Calculations
Pre-1982 Penny:
Trials Mass (g) Initial Level of water (mL)
Final level of water (mL)
Volume change
(mL)
15.39 g
15.36 g
20 mL
22 mL
22 mL
23 mL
2 mL
1 mL
23.5 mL 25 mL 1.5 mL 3
Number of Pennies
15.48 g
5
10
15
Mass (g)
15.39 g
30.75 g
46.23 g
Volume of Pennies (mL)
2 mL
3.5 mL
5 mL

Post-1982 Penny:
Trials Mass (g) Initial level of water (mL)
Final level of water (mL)
1 12.3 g 20 mL 21 mL
2 12.47 g
3
Number of Pennies
12.45 g
5 12.3 g
21 mL
23 mL
Mass (g)
23 mL
25mL
Volume change
(mL)
1 mL
2 mL
2 mL
Volume of Pennies (mL)
1 mL
3 mL
5 mL
10 24.77 g
15
Analysis
37.22 g
50
40
30
20
10
y = 9,2847x - 1,2797
0
0 1 2
Density: 9.28 g/mL
Percent Error: |9.28 – 8.96| / 8.96 = 3.57%
4 5 6

50
40
30
20
10 y = 7,0971x + 4,3782
0
0 1 2 3
Volume (mL)
4 5 6
Density: 7.09 g/mL - very close to zinc.
Percent Error : |7.09 – 7.13| / 7.13 = 0.56%
Conclusion/Discussion
Based on our data, we concluded that zinc is the unknown metal. This reflects the density of the remaining 97.5 percent of the penny that isn’t copper. The slope of our post-1982 penny graph was closest in value to the density of zinc, so we used the known value of the density of zinc to calculate our percent error, which was quite low. Any amount of error we may have encountered during our pre-1982 penny trials was likely due to a certain amount of water that may have splashed up onto the sides of the graduated cylinder when we placed the pennies in. This would skew the change in volume part of our data. Air bubbles can also form between pennies, which would throw off the value of the volume. However, our values of density for both the pre- and post- 1982 pennies were quite close to the accepted values, so we can reasonably conclude that the pre-1982 pennies were made of copper while the post-1982 pennies are made almost entirely of zinc.