Name ___________________________________ Date ________________ Class
_______________
Use Density to Date A Coin
A penny that has had its date scratched off is found at a crime scene.
The year the coin was minted is important to the case. A forensics
technician claims she can determine if the coin was minted before 1982
without altering the coin in any way. Knowing that pennies minted from
1962 to 1982 are 95% copper and 5% zinc, whereas those minted after 1982
are 97.5% zinc and 2.5% copper, hypothesize about what the technician
will do
Problem
How can you use density to determine whether a penny was minted before
1982?
Objectives
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Predict whether the pre- 1982 or the post-1982 pennies will
have a higher mass and volume
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Measure the volume and mass of pennies
-
Draw conclusions based on your analysis.
Materials
water
100-mL graduated cylinder small plastic cup
balance
Pre-1982
pennies (25)
Post-1982 pennies (25)
Metric ruler
pencil
graph
paper
graphing calculator (optional)
Safety Precautions
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Always wear safety goggles and a lab apron.
Pre-Lab
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Read the entire CHEMLAB.
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Prepare all written materials that you will take into the laboratory.
Be sure to include safety precautions and procedure notes.
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Review the equation for density. What would be the impact on density
of increasing mass while keeping volume constant?
-
Increasing the amount of the heavier element in an object would
increase the density of the object. Do you expect the pre-1982 pennies
or the post- 1982 pennies would have the higher density?
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What was the make-up of pennies before 1962? How would you expect the
density of pre-1962 pennies compare to pennies made post-1982? Between
1962 and 1982?
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Large objects cannot be placed in water to determine their volume.
Determine a procedure that could be used to calculate the density of
such an object.
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Review the equation to calculate percent error.
Procedure
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Read and complete the lab safety form.
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Record all measurements in your data table.
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Measure the mass of the plastic cup.
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Pour about 50-mL of water into the graduated cylinder. Record the
actual volume.
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Add 5 pre-1982 pennies to the cup, and measure the mass again.
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Add the 5 pennies to the graduated cylinder, and read the volume.
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Repeat steps 5 and 6 four times. After five trials there will be 25
pennies in the graduated cylinder.
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Cleanup and Disposal Pour the water from the graduated cylinder down
the drain, being careful not to lose any of the pennies. Dry the
pennies with a paper towel.
-
Repeat steps 3 through 7, using post-1982 pennies.
Data Table for the Density of a Penny
|
Trial
|
Mass of Pennies Added (g)
|
Total Number of Pennies
|
Total Mass of Pennies (g)
|
Total Volume of Water Displaced (mL)
|
1
|
|
5
|
|
|
2
|
|
10
|
|
|
3
|
|
15
|
|
|
4
|
|
20
|
|
|
5
|
|
25
|
|
|
Analyze and Conclude
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Calculate Complete the data table by calculating the total mass
and the total volume of water displaced for each trial.
-
Make and Use Graphs Graph total mass verses total volume for
the pre-1982 and post- 1982 pennies. Plot and label two lines on the
graph, one for pre-1982 pennies and one for post-1982 pennies.
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Make and Use Graphs Draw a best-fit line through each set of
points. Use two points on each line to calculate the slope.
-
Infer Examine the units for the slopes of the lines. Verifying
the slopes of the lines give you the density of the pre-1982 pennies
and density of the post-1982 pennies.
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Apply Can you determine if a penny was minted before or after
1982 if you know only its mass? Explain how the relationship among
volume, mass, and density support using a mass-only identification
technique.
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Error Analysis Determine the percent error in the density of
each coin.
Inquiry Extension
Compare your results with those from the rest of the class. Are
they consistent? If not, explain how you could refine your
investigation to ensure more accurate results. Calculate a class
average density of the pre-1982 pennies and the density of the
post-1982 pennies. Determine the percent error of each average.
Chemistry: Matter and Change • Chapter 2
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