Penny Isotopes
You will need some paper and a
calculator for this lab.
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.
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.’
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.
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.
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.)
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.
6. Unknown Sample
Mass of Unknown Sample:
# of Pennies In Unknown Sample:
Average Mass of Pennies in Unknown
Sample:
6. 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.
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?
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?
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 student’s most likely
mistake?