Penny Isotopes

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Penny Isotopes

• You will need some paper and a calculator for
  this lab.

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1. Mass of pennies

• Record the number of pennies in your container.
  List them by year then record the mass of each
  penny next to the year it was manufactured. Use
  the electronic balances and be sure to record ALL
  digits on the display. Some years may be
  duplicated record each penny individually.

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2. Pennies as an analogy for Isotopes(?)

• Pennies manufactured prior to 1982 were 95
  copper, 5 zinc.
• Pennies manufactured after 1982 are 5 copper,
  95 zinc.
• Pennies manufactured in 1982 are a mix of the
  older and newer types.
• So pennies seem to have 2 isotopes.

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3. Averages

• Calculate the average mass of the pennies in your
  sample which were made before 1982.
• Calculate the average mass of the pennies in your
  sample which were made after 1982.

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4. Average Atomic Mass

• Use the formula
• Average Atomic Mass ( of 1st Isotope X Mass of
  1st Isotope) ( of 2nd Isotope X Mass of 2nd
  Isotope)
• Calculate
• of pre-1982 pennies in your sample.
• of post-1982 pennies in your sample.

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4. Continued

• Use the average masses you calculated in Step 3
  and the of each isotope to calculate the
  Average Atomic Mass of a penny. (Remember to
  either change to a decimal 50 is .5, OR
  divide your final answer by 100. I prefer to
  change to a decimal at the beginning, since it
  leaves one less thing to forget.)

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5. Density

• Based on the average masses for pre- and
  post-1982 pennies and the composition information
  given in slide 2, which metal must be more dense
  Copper or Zinc?
• Use your textbook, the internet, or some other
  source to verify your answer. Record the actual
  densities of copper and zinc here.

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6. Unknown Sample

• Mass of Unknown Sample
• of Pennies In Unknown Sample
• Average Mass of Pennies in Unknown Sample

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Continued

• Average Mass ( 1 X Mass 1) ( 2 X Mass 2)
  .
• If there are ONLY 2 ISOTOPES, Then
• (1) (2) 1 100
• So (2) 1 ( 1)
• Substitute (1 - 1) in place of ( 2) in the top
  equation.

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6. Continued

• Now solve for ( 1.)
• 1 (1) (2)
• In the unknown sample
• What is the percent of pre-1982 pennies?
• What is the number of pre-1982 pennies?

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7. 1982

• A sample of ____ pennies minted in 1982 has a
  mass of ________ grams.
• If this sample is representative of all pennies
  minted in 1982 (this is doubtful) then what
  percent of pennies minted in 1982 were 95 copper?

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8. Error

• A student calculated the average atomic mass of
  their sample of pennies to be 255.92 grams.
• Is this a reasonable answer?
• If not, what was the students most likely
  mistake?