Name______Date______Per______ST#______

Pennium Lab: An Isotopic Discovery

A Isotope Discovery Experiment from HASPI Medical Chemistry Lab

Objectives

Find the number of each isotope of Pennium in the canister without looking.

Identify the percent abundance of each isotope

Materials

Bag containing 10 pre-1982 pennies and 10 post-1982 pennies

Pre-massed film canister filled with 10 pennies of unknown date

Balance

Scenario

Before 1982 pennies were made of nearly pure copper, however, as inflation rose, the cost of making a penny began to cost more than 1¢. This meant that people could take pennies out of their pocket, melt them down and sell the copper for more than the penny would have been worth. It also meant that the government would not be able to afford to make the amount of pennies needed, so they changed the penny so that the middle was made mostly from Zinc, an inexpensive metal, and only the outside was made of copper. Pennies now can model two "isotopes". The first “isotope” is a pre-1982 penny, which consists of 95% copper and 5% zinc. The other “isotope” is a post-1982 penny, which consists of 2.5% copper and 97.5% zinc.

The result is that a penny made before 1982 has a different mass than a penny made after 1982. We will consider these two isotopes of our element "Pennium" as we do the lab.

But what does this have to do with isotopes?

If you think about the fact that an isotope is an atom that acts in the same way chemically but has a different mass because of what is in the nucleus, you can see that we can use pennies, which can be used the same way but have a different mass due to their center, in order to model the relationship between isotopes. One definition is that isotopes are atoms of the same element that have nuclei with identical numbers of protons but have different numbers of neutrons.

We have to do one more thing to make this more realistic... hide the identity of our sample! Since there is no way to physically count up our isotopes we have to use math in order to find their percent abundance. In this lab we will hide our sample in a film canister to ensure that you are using the same technique that a scientist in a lab would use to find the percent abundance of each isotope (isotopic composition) in the sample... so remember - NO PEEKING!

Average Atomic Mass

When you look at the periodic table you might notice that the masses are not whole numbers. If each proton and neutron each have masses of 1 amu (atomic mass unit), what might cause an element to have a listed mass that is not a whole number? The different masses of each isotope contribute to the average mass of a sample. If you consider carbon, which has an average atomic mass of 12.01, this is because almost all of the carbon atoms have a mass of 12, however there are some naturally occurring carbon atoms with masses of 13 or 14, which increases the average mass of all carbon atoms to 12.01.
Procedure: Part A Learning About your Pennium Isotopes

Our first goal is to investigate each isotope individually and try a few problems to prepare us to work with our mystery sample.

  1. Obtain a sample of ten pennies (5 pre-1982 and 5 post-1982).
  2. Find the mass of each of 5 of the pennies and record it in the correct column below. Then find the average of the masses for each "isotope" of Pennium.
  3. Try the exercise with a total of 10 pennies. Mass them together, & then find the average mass.
  4. Compare your average mass answers to step 2 and step 3. Which should be more accurate?

Pre-1982 Penny Date / Mass (g) / Post-1982 Penny Date / Mass (g)
Average Mass pre-1982: / Average Mass post-1982:
Mass of 10 pre-1982 pennies / Mass of 10 post-1982 pennies
Average mass of pre-1982 penny / Average mass of post-1982 penny
Question & Work / Answer
5. Calculate the expected mass of a combination of 2 old pennies plus 4 new pennies by using the more accurate of your average masses above. / g
6. Find the average mass of the pennies in the sample in question 5. / g
7. Now mass a sample of two old and four new pennies. Record the mass. / g
8. Divide your answer by six to find the average mass of a penny in your sample from step 7. / g
9. Compare your answer for number 6 to your answer for number 8. Is the “weighted average mass” closer to the mass of an old penny or a new penny? Why?
10. How is the weighted average mass related to the atomic mass found on a periodic table?

Procedure Part B: Discovering the contents of your mystery sample

Return your sample of ten pennies from part A to your teacher. Get a canister of pennies.

  1. Procedure
/ Data
1. Don’t open your sample. Record its identifying letter from the top of the canister / Identifying letter
2. Record the mass of the empty film canister, this measurement is in grams and is on the label of the canister. / Mass of empty canister
3. Use the balance to determine the mass of the sealed canister with pennies / Mass of sealed canister & pennies
4. Subtract the mass of your canister from the total mass and record the mass of the pennies / Mass of pennies in the canister
5. Find the average mass of Pennium in your sample / Average mass of Pennium in sample

Return film canister to your teacher, if asked to do so.

Calculations: Find the abundance of each isotope of Pennium in your container
  • You know that the total number of pennies is ten,
  • Therefore you can say that there are x old pennies plus 10 – x new pennies.

If we multiply the amount of old pennies by its mass we can find the total mass of old pennies. This would look like (Mass of one old penny)(X) = mass of all old pennies
If we multiply the amount of new pennies by its mass we can find the total mass of new pennies. This would look like (Mass of one new penny)(10-X) = mass of all new pennies
Finally, if we add the mass of all new pennies and the mass of all old pennies we get the total mass of pennies in the canister.
Here is the equation, using the information above, that relates your values from the lab:
(X)(mass of old penny) + (10–X) (mass of new penny) = total mass of pennies
1.Plug your found values into the equation and solve for x
What did x stand for?
2. How many old pennies
are in the canister? / 3. How many new pennies
are in the canister?
4. What is the percentage
of old pennies? / 5. What is the percentage
of new pennies?
Conclusion
1. Explain why these pennies are a good model for isotopes:
2. Describe two shortcomings of the Pennium model for isotopes
3. Did your average mass of pennies match the mass of any one penny? Explain why or why not:
4. Explain the relationship between the average mass you found for your pennies in the canister and the average atomic mass of an element on the periodic table.
Practice
1. New element Q has three isotopes Q-12 is 62.10% abundant, Q-13 is 2.43%abundant, and Q-14 is 35.47% abundant. Determine the average mass of elementQ, which is the weighted average of the 3 isotopes (same as we do it for all of the isotopes of each element on the actual periodic table). Show all work!
2 A new element was discovered on planet X. The element exists as two differentisotopes, E-21 and E-23. Which isotope is more abundant if the average mass is21.7g. Explain.
3. Calculate the average atomic mass of titanium (weighted average mass of the Ti isotopes). The five titanium isotopes have atomic masses and relative abundances of 45.953amu (8.00%), 46.952amu (7.30%), 47.948amu (73.80%), 48.948amu (5.50%) and 49.945amu (5.40%).