As you learned from the “Hollow Penny” activity, pennies minted before 1982 are pure copper. Newer pennies are actually almost entirely composed of zinc, but the thin coating of copper on the outside makes new pennies look very much like they are made of copper. Copper and zinc are different elements and therefore have different
density values. By determining the
density of each type of pennies, the composition of the metal can be confirmed. Older copper pennies should have a different
ratio of
mass to
volume (density) than zinc pennies.
Prelab activity
Use the internet to find the theoretical
density of zinc and copper. Look for units of grams/cm
3. Use this information to make a
hypothesis for the experiment.
Density of copper = ____________g/cm
3
Density of zinc = ____________ g/cm
3
Procedure
- Using the mint dates, separate out the pennies into a copper and a zinc pile. You will need 15 pennies of each type.
- 2. Place 50.0 mL of water into a graduated cylinder. Record the initial water level of water as 50.0 mL.
- 3. Put the cylinder on the balance. Record the initial mass of the cylinder and water.
- Add 3 copper pennies to the cylinder. Notice that the water level rises. Record the final water level. The volume of the pennies can be determined by water displacement (i.e. by taking the difference between the volumes).
- Put the cylinder on the balance. Record the mass of the cylinder, water and the pennies.
Find the mass of the coins by subtraction.
- Add three more pennies, so that there is a total of 6 coins in the cylinder. Record the volume and the mass.
- Keep adding the pennies, in groups of 3, until you have put all 15 copper pennies into the water.
- When finished with the copper pennies, repeat the process using zinc pennies.
Data for the copper pennies
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Trial
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Volume of water and coins (mL)
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3 coins
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50.0
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6 coins
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50.0
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9 coins
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50.0
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12 coins
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50.0
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15 coins
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50.0
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Data for the zinc pennies
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Trial
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Volume of water and coins (mL)
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3 coins
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50.0
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6 coins
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50.0
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9 coins
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50.0
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12 coins
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50.0
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15 coins
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50.0
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Calculate the density for each trial.
Since you have five
density values, find the average
density for each metal.
Average =
Compare the theoretical
density to the average experimental
density by calculating the % error.
x 100
Using your graphing calculator or LoggerPro, create a
graph of
mass (y-axis) versus
volume (x-axis) for each metal. You will plot the five
data points for each metal. Calculate the
slope of each line. The
slope represents the mass/volume or the
density of the metal. Both lines can be plotted on the same
graph so that the results can be easily compared.
Print out a copy of the
graph to include in your lab report. Be sure to write the
slope of each
line on the graph.
Conclusions
- State your results. What is the average experimental density for each metal?
- State the theoretical value.
- State the % error.
- Think about and suggest at least two valid sources of error.
Suggest at least two ways to improve the experiment.