Isotope PenniesLab

Introduction:

Isotopes are atoms, of a given element, that have the same number of protons, but a different number of neutrons in the nucleus. For example: C-12 and C-14 are carbon isotopes. They each have 6 protons (atomic # = 6), but one has 6 neutrons while the other has 8 neutrons. These carbon atoms have similar chemical and physical properties, but they have different masses due to their differing number of neutrons. Some carbon isotopes weigh 12 atomic mass units (amu's) and other carbon isotopes weigh 14 amu's. The Average Atomic Mass of all carbon

isotopes is 12.011 amu's. Can you predict which carbon isotope is more abundant based on the average mass of a carbon sample?

The mass of a penny has changed in 1982 as they changed from100% copper to about 4% copper and 96% zinc. As a result, penny made after 1982 has lower mass than those made prior to 1982. Just like Isotopes! In the lab, we will be treating pre-1982 and post-1982 pennies as two different"Penny Isotopes".

Purposes:

  • Confirm the validity of the formula for calculating average atomic mass.
  • Determine the composition of the unknown isotope penny mixture.

Materials:

  • Pre-1982 and post-1982 pennies
  • Electronic balance
  • Unknown isotope penny mixture
  • Calculator on laptop or handheld

Pre Lab Assignment: read pages 100-104 in textbook. Do example problems 4-2 and 4-3 and practice problems on p. 104. See ppt on wires for assistance; Atoms and Isotopes

Pre-Lab Questions:

  1. What is the definition of isotope?
  2. What is the definition of mass number?
  3. What is the definition of atomic mass?
  4. What do the two kinds of pennies represent in this lab?
  5. How do the pennies differ? What is the difference between pre and post 1982 pennies?
  6. How do isotopes differ?
  7. What do the pennies have in common? What do isotopes have in common?
  8. What is the formula for calculating average atomic mass of elements?
  9. What is the % abundance?
  10. Construct proper data tables. Table for each section of measurement.

Recommendation: table for part A

Table for part B steps 1-3, table or section of previous table for section 4

Table for Part C

Table for part D- cleaning pennies

Data Analysis/Calculations section should include formulas to be used as well as tables to include answers in calculations. See ppt and text

Procedures:

Part A: Average Mass for Isotope Pennies

Be careful not to mix the pre and post 1982 penny collections.

  1. Carefully mass each of the 10 pre-1982 pennies on the electronic balance. Record in the data table. Leave the pennies on the lab bench.
  2. Mass all of the ten pennies and record in the data table.
  3. Observe the appearance of those pennies-record what is observed.
  4. Return pennies to proper container.
  5. Repeat step 1 for the 10 post-1982 pennies. Record in the data table. Make sure no mixing between the two kinds of pennies. Leave the pennies on the bench.
  6. Mass all ten pennies and record in the data table.
  7. Observe the appearance of those pennies and record.
  8. Return pennies to its container.

Part B: Isotope Pre and Post 1982 Penny Mixtures with Known Composition

  1. Take 1 penny from the pre-1982 container and put it on the balance. Take 9 pennies from the post-1982 container and put on the balance with the 1 pre penny. Make sure there is no mixing between the two kinds of pennies. Record the total mass of the ten pennies in the data table.
  2. Take 1 post-1982 penny off the balance and put it back into corresponding container. Take 1 pre-1982 penny from its container and onto the electronic balance. Record the mass in the data table. Note: keep the total number of pennies at 10.
  3. Repeat step 2 for the following pre-1982/post-1982 penny combinations: 3/7, 4/6, 5/5, 6/4, 7/3, 8/2, 9/1. Record each mass in the data table. Make sure there is no mixing between the two kinds of pennies.
  4. Take any number of pre-1982 pennies (less than 15) and post-1982 pennies (less than 15). Record the exact number of each pre and post pennies used for this measurement. Measure the total mass on the electronic balance. Record in the data table.
  5. Return all pennies to corresponding containers after the final measurement.

Part C: Mystery Isotope Pennies Mixture

  1. Take one sample canister from the instructor. Record the unknown sample ID in the data table.
  2. Mass the canister with the pennies on the electronic balance. Record in the data table.
  3. Empty the canister. Count number of pennies and record in the data table.
  4. Mass the empty canister and record in the data table.
  5. Put pennies back to the canister and return it to the instructor.

Part D: Cleaning Dirty Pennies

  1. Measure between 10 to 15 mL vinegar. Record the volume in the data table.
  2. Pour all the vinegar into a clean 150-mL beaker.
  1. Add small amount (spatula tip full) of table salt into the beaker containing vinegar. Stir briefly with glass rod.
  2. Select two dirty pennies from either pre- or post-1982 pile. Soak in the vinegar/table salt mixture for 3 minutes. Stir occasionally and observe. Record in the data table.

Data: Any design that is organized, logical is acceptable

Recommendation:

Table for part A

Table for part B steps 1-3, table or section of previous table for section 4

Table for Part C

Table for part D- cleaning pennies

Data Analysis/Calculations section should include formulas to be used as well as tables to include answers in calculations. See ppt and text

Sample Calculations: Formulas used (show all math & units)

Data Analysis:

Part A: Average Mass for Isotope Pennies

  1. Calculate total mass for 10 pennies. [Add the masses of 10 pennies from each measurement.]
  2. Calculate the average mass for each penny. [Take the total mass, from step 1, divide that by the number of pennies, 10.]
  3. Calculate the average mass for each penny from the mass of 10 pennies. [Take the mass of 10 pennies measured at once, divide that by the number of pennies, 10.]
  4. Q: Are those two average mass for each penny the same or different? [Compare the two averages.]
  5. Q: Can those two types of pennies easily distinguishable by simple visual inspection? Why yes? Or why not?

Data Table ______

Part B: Isotope Pennies Mixtures with Known Composition

  1. Calculate average mass of each penny in the mixtures. Record in the data table. [Take the total mass of 10 pennies, divide that by 10 for each combination of pre- and post-1982 pennies.]
  2. Calculate the weighted average for each penny mixture by following the steps below:
  3. For each mixture, there are total 10 pennies. Calculate the fractions of pre-1982 pennies with respect to the total number of pennies (10). Calculate the fractions of post-1982 pennies. Record in the data table.
  4. Take each fraction, multiply with the average mass of corresponding kind of penny (from Part A). Record the results in the data table.
  5. Add the two values and record the results in the data table as the weighted average mass.

Example: average mass for the pre-1982 penny is 4.12 g; average mass for the post-1982 penny is 2.95 g. There are 4 pre-1982 pennies and 6 post-1982 pennies. Fractions are 4/10 (0.4) for pre-1982 and 6/10 (0.6) for post-1982 pennies. Multiply 4.12 g by 0.4 and 2.95 g by 0.6, respectively. Add the results (1.65 g + 1.77 g) to get the weighted average of 3.42 g. [Calculate weighed average according to the instructions listed here. Key elements of the calculation are the average mass for pre- and post-1982 pennies obtained from Part A and number fractions for each combination.]

  1. Compare the weighted average mass with the measured value (step 1). Draw firm conclusions. [Compared calculated value, step 2 with measured value, step 1 and draw conclusions.]

Part C: Mystery Isotope Pennies Mixture

  1. Calculate the total mass of the pennies.
  2. Calculate the average mass of each penny.
  3. Estimate the composition of the penny mixture (# of pre-1982 pennies and # of post-1982 pennies). State the reasoning in complete sentences.

Data Table ______

Conclusions and Discussions: (the post lab questions are the entire conclusion requirement)

Post-lab Questions:

  1. Predict the average mass for each penny where the sample contained 25 pre-1982 pennies and 35 post-1982 pennies. Note: use the average mass of pre-1982 and post-1982 pennies from Part A of the lab. The weighted average approach should be used. Then test it out by measuring the total mass of 25 pre-1982 and 35 post-1982 pennies, calculate the average mass and compare to the predicted value.
  2. Rubidium has two common isotopes, and . If the abundance of is 72.2% and the abundance of is 27.8%, what is the average atomic mass of rubidium?
  3. Titanium has five common isotopes: (8.0%), (7.8%), (73.4%), (5.5%), (5.3%). What is the average atomic mass of titanium?
  4. How are the qualitative observations, made in Part D, classified? What are they?
  5. Extension question: what is the nature of the residue on pennies? Why does the vinegar/table salt mixture works in cleaning dirty pennies? Explain in details.

What have you learned in this lab?

Discuss what you preferred and did not prefer about this lab. Did this exercise help yourunderstanding of isotopes and method of determining average atomic mass?

Why study isotopes?

How are isotopes used and the knowledge of them, used in the medical field? In industry?In research or other fields?