GO IN DEPTH

# Isotopes of Pennies

By Roman Oleinik (Own work) [CC-BY-SA-3.0], via Wikimedia Commons

### Introduction

You will do a lab that will deal with isotopes, mass number, and atomic mass.

Before you begin your work in the lab, try to explain these terms in your own words. After you
have finished the lab, you will have a chance to revise your explanations based on what
you have learned in the activity.

Isotope
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Mass number
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Atomic mass
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In 1982, the United States government changed the way it minted pennies. Before 1982,
pennies were made of 95% copper and 5% tin. Now they are made of zinc coated with
copper. Because they weigh different amounts (have different masses), we can call
them isotopes of pennies.

• What do the two kinds of pennies represent in this exercise?
• How do the pennies differ? How do isotopes differ?
• What do the pennies have in common? What do isotopes have in common?

Part A:

1. Obtain a sample of ten pennies.
2. Weigh several pre-1982 (old) pennies and record their average mass. ______g
3. Weigh several post-1982 (new) pennies and record their average mass. ______g
4. Calculate how much three old pennies plus seven new pennies should weigh. ______g
5. Divide your answer for number 4 to find the weighted average mass of the pennies in the sample containing three old plus seven new pennies. ______g
6. Now weigh your sample of three old and seven new pennies. Record the mass. ______g
7. Divide your answer for number six by ten to find the average mass of a penny in your sample. ______g
9. average mass closer to the mass of an old penny or a new penny? Why?
10. How is this weighted average mass related to atomic mass?

Part B

1. Obtain a sample containing six old pennies and four new pennies.
2. Using the mass of an old penny and a new penny from part A above, calculate a weighted average mass for this sample of pennies. You need to find the mass of all ten pennies and divide by ten to find the weighted average mass. ______g
3. Now weigh your sample of pennies. Record the mass. ______g
4. Divide the mass of your sample of ten pennies by ten to find the actual average mass of a penny in this sample. ______g
6. average mass closer to the mass of an old penny or a new penny? Why?

Part C: The Mystery Sample

1. Return your sample of ten pennies from part B to your teacher. Get a canister of pennies.
2. Don’t open it. Record its identifying number or letter:______
3. Record the mass of the empty film canister, which is on the label of the canister. ______g
4. Weigh the sealed film canister containing ten mixed pennies. ______g
5. Return the canister to your teacher.

Calculations:

1. Calculate the number of old and new pennies in your canister:
2. Since the total number of pennies is ten, we can say that there are x old pennies plus 10 – x new pennies. The total mass of the pennies (canister with pennies minus the mass of the canister) is useful here.
3.  X times the average mass of an old penny plus (10 – x) times the average mass of a new penny equals the total mass of the pennies in the canister. Set up an equation and solve for x. Then you will know how many old pennies are in your canister. Subtract that number from ten to find the number of new pennies that are in your canister.