Chemistry+Intro+Lab



= = = **Use Density to Date a Coin** =


 * Background:** A penny that has had its date scratched off is found at a crime scene. The year the coin was minted is important to the case. A forensics technician claims she can determine if the coin was minted before 1982 without altering the coin in any way. Knowing that pennies minted from 1962 to 1982 are 95% copper and 5% zinc, whereas those minted after 1982 are 97.5% zinc and 2.5% copper, hypothesize about what the technician will do.


 * Purpose:**
 * To determine the density of a pre and post 1982 penny.
 * To use density to determine if a penny was minted before or after 1982.

water 100ml graduated cylinder massing dish pre-1982 pennies (25) post-1982 pennies (25)
 * Materials:**


 * Procedure:**
 * 1) Pour about 50ml of water into the graduated cylinder. Record the actual volume.
 * 2) Measure the mass of a massing dish and add 5 pre-1982 pennies to the massing dish. measure and record the mass.
 * 3) Add the 5 pennies to the graduated cylinder, and read the volume.
 * 4) Repeat the previous steps until all five trials are completed and 25 pennies are in the graduated cylinder.
 * 5) Repeat the previous steps using post-1982 pennies.


 * Post Lab Questions and Activities:**
 * 1) Graph total mass versus total volume for the pre-1982 and post-1982 pennies. Plot and label two sets of points on the graph, one for pre-1982 pennies and one for post-1982 pennies.
 * 2) Draw a best-fit line through each set of points. Use two points on each line to calculate the slope.
 * 3) What do the slopes of the lines tell you about the two groups of pennies?
 * 4) Can you determine if a penny was minted before or after 1982 if you know only its mass?
 * 5) Explain how the relationships among volume, mass and density support using a mass-only identification technique.
 * 6) Determine the percent error in the density of each type of coin.