Modeling Isotopes PreLab - Honors Chemistry

Modeling Isotopes PreLab - Honors Chemistry

Honors ChemistryLab: Modeling IsotopesIntroductionThe defining characteristic of an atom of a chemical element is the number of protons in its nucleus. Agiven element may have different isotopes or nuclei with the same number of protons but different numbers ofneutrons. For example, C-12 and C-14 are two isotopes of carbon and the nuclei of both atoms must contain sixprotons. However, C-12 has six neutrons while C-14 has eight neutrons. In general, it is the number of protonsand electrons that determines chemical properties of an element. THUS, the different isotopes of an element areusually chemically indistinguishable from each other. These isotopes have different masses though because oftheir different composition.Is a penny always worth 1 cent? During the late 1970’s and early 1980’s, the U.S. government had adilemma. Copper had risen in value and the amount of copper that was actually in a penny was worth morethan 1 cent. The cost of minting each penny was now too high and something needed to be done. Between1962 and 1982, pennies were made of brass, an alloy composed of 95.0% copper and 5.0% zinc. Since 1983,U.S. pennies have been minted with a zinc core and only plated with copper. Zinc is a relatively inexpensivemetal, so the majority of the penny’s mass is actually due to the zinc. These newer pennies contain 2.5%copper and 97.5% zinc. Because of this, pennies have a different composition and different masses. This partof this activity uses a mixture of pre- and post-1982 pennies to represent the naturally occurring mixture of twoisotopes of this imaginary element "coinium". Using the pennies allow us to model one way in which scientistscan determine the relative amounts of different isotopes in a sample of element.You will be given a sealed container that holds a mixture of ten pre- and ten post-1982 pennies. Yourcontainer might hold any particular combination of the two "isotopes" of coinium. Your task is to determine theisotopic concentration of the element coinium WITHOUT opening the container.HOW are we to do this? Rely on your powers of algebra - math is a powerful tool that scientists use.For starters, we know that there will be ten pennies in the container so we can let x represent the total number ofpre-1982 pennies and (10 - x) represent the number of post-1982 pennies. Additionally, the TOTAL MASS ofpre-1982 pennies equals the number of pre-1982 pennies multiplied by the mass of one pre-1982 penny.Likewise, the total mass of post-1982 pennies equals the number of post-1982 (10-x) pennies multiplied by themass of one post-1982 penny. Now, put your algebra to work:mass all the pennies = (x · mass pre-1982 penny ) + [(10 - x) · mass post-1982 penny ]Our goal for the first part of the lab is to find the value of x, the number of pre-1982 pennies in thecontainer. Once that is known we will have figured out the composition of the 10-penny mixture withoutopening the container! Don't cheat!!! We will then use the densities of copper and zinc (with a little geometry)to help us verify the masses of pre-1982 and post-1982 pennies.1

Pre-Lab1. On your analysis sheet, make “Data Table 1” to record the total mass (g) of a group of ten pre-1982 penniesand then a group of ten post-1982 pennies. Include a place to record the average masses of each type ofpenny as well. You will also need another table, “Data Table 2,” to record the identity of your container,mass of empty container, the total mass of the container and pennies, and the mass of the pennies inside thecontainer. A space has been provided for you to make this table on your analysis sheet.Procedure2. Obtain 10 pre-1982 and 10 post-1982 pennies, and two sealed containers with pennies inside.3. Record the masses of each group of pre-1982 and post-1982 pennies on Data Table 1. Determine theaverage mass of each type of penny and record it as well.4. Record the identity of your sealed penny containers, along with the empty container masses (provided byyour instructor) on Data Table 2.5. Record the total mass of the sealed container and pennies on Data Table 2.6. Calculate the mass of the 10 pennies inside the container. Record on Data Table 2.2

Honors ChemistryLab: Modeling IsotopesData Table 1: Penny InformationName _______________________Partner _____________________Data Table 2: Container InformationLab Analysis1. Calculate the number of pre- and post-1982 pennies in the element "coinium". You will do a separatecalculation for each container. (Hint: See Introduction) Show your work here:Trial 1 Container _______:Number of pre-1982 pennies in container used in trial 1: ______________________Number of post-1982 pennies in container used in trial 1: ______________________Trial 2 Container ______:Number of pre-1982 pennies in container used in trial 2: ______________________Number of post-1982 pennies in container used in trial 2: ______________________3

7. Calculate the theoretical mass of a pre-1982 and a post-1982 penny.a) The density of copper is 8.96 g/cm 3 and that of zinc is 7.13 g/cm 3 . Using the composition given in theintroduction, calculate the DENSITIES of a pre- and a post-1982 penny. (2 separate calculations)b) A typical penny has a diameter of 1.905 cm and a thickness of 0.124 cm. What is the volume in cm 3 ofa typical penny? (Hint: volume has something to do with πr 2 ) (only 1 calculation)c) Using the density and volume values, calculate the theoretical mass of a pre-1982 penny and thetheoretical mass of a post-1982 penny.(2 separate calculations)d) Compare the theoretical masses from the question 7c) to your average masses from Data Table 1 bycalculating a percent error for the pre-1982 and post-1982 pennies. (2 separate calculations)8. In 1982 the composition of the penny started to change. Some foundries were producing pennies with theolder composition while other foundries were having the machinery re-tooled to make the zinc core penny.This resulted in some 1982 pennies having zinc cores while others were the older brass alloy. List three (3)different properties of these metals that could be used in the laboratory to determine the composition of apenny. (Do not use mass, volume, or density; we already used those properties in this lab.)5

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