Count Your Pennies and Make Some Cents

Objective: To investigate and use an equation with two variables

Materials: Paper cups

Pennies with dates before and after 1982

Triple beam balances

Procedure: The student steps are below. Have the students complete these steps after giving them the story.

1. Obtain 10 pre-1982 pennies. Measure and record the mass of this group of pennies.

2. Trade data with 2 other groups in the class, and record their data, too.

3. Total all masses and divide by the total number of pennies weighed, to get the average mass of a pre-1982 penny. Label this the average mass of the Isotope pre-1982.

4. Repeat for the post-1982 pennies.

5. Obtain a container of 10 mixed pennies from your instructor. Record the code name that is on the container.

6. Note that the mass of the container is written on it.

7. Mass the whole container and subtract out the mass of the container itself, to obtain the mass of the pennies alone.

8. Calculate the average penny mass.

9. Figure out how many pre-1982 and post-1982 pennies you have in the container. Of course, you may not open the container. (Put this work in your data table which will be on a separate paper.)

10. What percent of each type of penny do you have?

Count Your Pennies and Make Some Cents

We will carry out this activity using pennies. Picture a pile of pennies. They all look pretty much the same, don’t they? Yet, pre-1982 pennies have a different composition and thus a different mass than pennies minted after 1982. So pennies can serve as an analogy to isotopic atoms; they may look and act basically the same, yet their compositions and mass can vary.

Read this excerpt from a Congressional report about “The Proposed Change in the Penny”, before proceeding.

In April of 1980 the United States Mint made the decision to change the alloy that was used in the making of pennies as prescribed in 1974 by an act of Congress. The coins were being made of an alloy containing 95% copper and 5% zinc. The problem was that the price of copper was on the rise and the cost of producing pennies was approaching the face value of copper. It was determined that, when the price of copper reached $1.12 per pound, pennies would be worth less than the copper that was being used to produce the pennies.

During a Congressional Hearing on March 31, 1981, there were several examples of penny hoarding described. Many stores and banks were offering as much as 20% more for turning in pennies. There was also a case involving the Secret Service and a major electrical firm that was taking 1-cent coins and turning them into washers. It was cheaper to take the coin, put a hole through it, and use it as a washer, than it was to start with fabricated material and manufacture that washer.

It was decided that the alloy used to make the 1-cent coin should be changed so it would now be composed of 99.2% zinc and .8% copper barrel electroplated with a minimum thickness of 0.0002 in. of copper. This would give an overall composition of 97.6% zinc and 2.4% copper while not changing the size or wearing properties of the coin. The new penny would be lighter in mass (from 3.1g to 2.5g) and save 20 - 50% in production costs.

As you proceed in this lab, data must be collected. A table is not provided so you must design your own. Use a separate piece of paper and make sure to record all measurements.