Using Pennies and Good Sense

CHEM 211 Laboratory

Using Pennies and Good Sense

Reference: "An Experiment in Thinking Scientifically", D. J. Sardella, J. Chem. Ed. 69, 933 (1992).

Purpose: The purpose of this exercise is to determine experimentally the variation in mass of pennies both within a single mint year and over a range of years.

Equipment: Balance, pennies

Procedure and Observations: Choose a year and, using a single balance, determine the precise mass (in grams) for each of your pennies (up to a maximum of 10), all from the same mint year. Record all your data in your laboratory notebook as you weigh the pennies.

Data: After you have recorded all the masses, enter your own data into the appropriate Web-based data entry form, using one of the computers in the lab. Be sure to enter the masses to the full precision of the balance, i.e. to the nearest 0.001 g.

Results: Outside of class, retrieve your data in spreadsheet form (along with that of the entire class) from: http://chem.gmu.edu/results/. Then use Excel to calculate the average mass and the standard deviation for each year, using the AVERAGE() and STDEV() spreadsheet functions. Finally, calculate the standard deviation for ALL the pennies, considered as a single large sample.

Use Excel to prepare a chart showing the average mass vs. year. Also, prepare a second chart showing the standard deviation vs. year.

Discussion: In your discussion section address the following questions:

·  How does the standard deviation among the pennies in a given year compare with the standard deviation among all the pennies (all years)? What can you conclude from this?

·  Describe in one sentence any trend observed in the variation of average mass with mint year.

·  Describe in one sentence any trend observed in the variation of the standard deviation with mint year.

·  Give a possible explanation for these trends and discuss how your observations support this explanation.