The Abundance of Nuclides A Lab Simulation Using Pennies Eric Hendrickson Presque Isle High School, 16 Fort Street, Presque Isle, ME 04769 In the study of chemistry there are several teaching modules where the performance of laboratory experiments is difficult or even impossible in many chemistry labs. The study of the relative abundance of nuclides can be one of thosemodules. The lab titled "It's Beanium!!" by Gregory C. Miller in the Journal of Chemical Education1 shows how the relative isotope type calculations can be made knowing the percent composition. However, calculations involving the determination of the relative abundance of nuclides have been hard to show in the form of a lab. That is, until 1982 when the United States government changed the composition of the penny. This became the perfect chance to do just such an experiment that would relate to this type of module including good, sound lahoratory calculations. Hlstorlcal Background 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 facevalue of the copper. I t was determined that, when the price of copper reached $1.12 per pound, pennies would be worthless than the copper that was being used to produce the pennies. During the Congressional Hearing: The Proposed Change in the Penny, March 31,1981, there were several examples of penny hoarding. Many stores and banks were offering as much as 20% more for turnine- in ~. e n n i e sThere . was also a case invol\.ing the Secret Service and a major elertrical firm that was taking 1-cent coins and turnina them into washer^.^ I t was cheape;to take a 1-cent coin, put a hole through it, and use it as a washer, than it was to start with the fabricated material and manufacture that washer. It was decided that the alloy used to make the 1-cent coin would be changed so that the alloy would be composed of 99.2% zinc and 0.8% 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 of wearing properties of the coin. The new penny would be about 19%lighter in mass (from 3.1 g to 2.5 g) and save from 20% to 50% in production costs. General Laboratory Procedure The lab is relatively easy to perform. The change in the mass of the pennies allows the mass difference to be measured on a centigram balance. The student should obtain a sample of 20 to 30 clean coins that are either pre-1982 or post-1982. The coins should be heated slightly to remove moisture, but care should he taken when cleaning the coins not to oxidize them with an excess of heat. The sample should not contain any foreign coins, because the alloy comoosition is different, or anv 1982 coins because the change &as made during that year; The mass of these coins should be taken to the nearest 0.01 g and recorded. The student should then obtain a random sample of 30 to 40 coins making sure to exclude all foreign and 1982 coins. 986
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The mass of this sample should be takento the nearest 0.01 g and recorded. The student may want then to determine the number of post-1982 and pre-1982 coins in the random sample to use to calculate percentage error from the calculated data. Sample Data Number of pre-1982 coins Mass of the pre-1982 coins Number of coins in the random sample Mass of random sample of coins Density of zinc3 Density of copper3
29 coins
89.31g 35 coins 105.50 g 7.14 g/cm3 8.94 g/cm3
Derlved Values Density of the pre-1982 coins (8.94 g/cm3)X (0.95) + (7.14 g/m% (0.05) = 8.85 g/cm3 Density of the post-1982 coins (8.94 g/cm3)X (0.024) + (7.14 g/cm3)X (0.976) = 7.18 g/cm3 The volume of a single coin (assuming that all coins are the same size) Volume of the random sample 35 coins X 0.348 em3/coin = 12.18 em3 Density of the random sample Percentage of pre and post coins
+
(8.85g/cm3)X (X) (7.18 g/m3) X (100X)1100= 8.66 g/cm3 X = 88.62% (pre-1982 coins) (100X) = 11.38%(post-1982coins) Number of pre-1982 coins 0.8862 X 35 coins = 31.02 pre-1982 coins Number of post-1982 coins 0.1138 X 35 coins = 3.98 post-1982 coins DiscuDolon Although all atoms of the same element have the same number of protons, the number of neutrons that they have
'Miller, G. C. J. Chem. Educ. 1981, 58, 507. *United States House of Representatives Subcommittee on Consumer Affairsand Coinageofthe Committee on Banking, Financeand Urban Affairs. Hearing on "The Proposed Change in the Penny"; 97th Cong.. Serial Number 97-4: Washington. 1981: p 19. CRC Handbook of Physics and Chemistry.
may differ. These different forms of the same element are called isotopes. An example of this, taken from the CRC Handbook of Physics and Chemistry, would be chlorine with its different isotopes containing from 15 neutrons in C132to 23 neutrons in C P . If there are several isotopes of an element in nature, they are found in very specific amounts (Cl3L75.53%, CP724.47%, with negligible amount for all the other isotopes). The atomic mass is the average of the isotopes massed according to their percentage of relative abundance in nature whirh in the rase of chlorine would be 35.453. The lab can he completc in about 20 to 30 min. The sample analysis used above was taken directly from a students iah report. The lab shows extremely well that, while two coins ( p e n n ~ P ' ~and - ' ~pennyPost-'82), ~ isotopes, may look the same,
there are major differences in their composition (atomic structure). I have used this lab with upper and lower levels of students with a great deal of success. Both levels are able to achieve a great deal of success with the lab and a good understanding of the processes used for determining the atomic mass. The students seem to take a great deal of pleasure and pride in their work when they have made careful measurements and are rewarded with correct answers and a sound conclusion. This lab is mathematically calculated on paper in the lab first, then the work can be easily verified using software developed by the author. But in lower-level chemistry classes, students make measurements and use template-type software only to find derived values. For further information on these programs, please contact the author.
Volume 65 Number 11 November 1988
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