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As you learned from the “Hollow Penny” activity, pennies minted before 1982 are pure copper. Newer pennies are actually almost entirely composed of zinc, but the thin coating of copper on the outside makes new pennies look very much like they are made of copper. Copper and zinc are different elements and therefore have different density values. By determining the density of each type of pennies, the composition of the metal can be confirmed. Older copper pennies should have a different ratio of mass to volume (density) than zinc pennies.

Prelab activity

Use the internet to find the theoretical density of zinc and copper.  Look for units of grams/cm3. Use this information to make a hypothesis for the experiment.

Density of copper = ____________g/cm3

Density of zinc =     ____________ g/cm3

Procedure

1. Using the mint dates, separate out the pennies into a copper and a zinc pile. You will need 15 pennies of each type.

2. 2. Place 50.0 mL of water into a graduated cylinder. Record the initial water level of water as 50.0 mL.

3. 3.  Put the cylinder on the balance. Record the initial mass of the cylinder and water.

4. Add 3 copper pennies to the cylinder. Notice that the water level rises.  Record the final water level. The volume of the pennies can be determined by water displacement (i.e. by taking the difference between the volumes).

5. Put the cylinder on the balance. Record the mass of the cylinder, water and the pennies.

Find the mass of the coins by subtraction.

6. Add three more pennies, so that there is a total of 6 coins in the cylinder. Record the volume and the mass.

7. Keep adding the pennies, in groups of 3, until you have put all 15 copper pennies into the water.

8. When finished with the copper pennies, repeat the process using zinc pennies.

Data Tables

Data for the copper pennies

 Trial Mass of cylinder with water and coins (grams) Mass of cylinder with water (grams) Mass of coins (grams) Volume of water and coins (mL) Initial volume (mL) Volume of coins (mL) Density(g/mL) 3 coins 50.0 6 coins 50.0 9 coins 50.0 12 coins 50.0 15 coins 50.0

Data for the zinc pennies

 Trial Mass of cylinder with water and coins (grams) Mass of cylinder with water (grams) Mass of coins (grams) Volume of water and coins (mL) Initial volume (mL) Volume of coins (mL) Density(g/mL) 3 coins 50.0 6 coins 50.0 9 coins 50.0 12 coins 50.0 15 coins 50.0

Data Analysis

Calculate the density for each trial. Since you have five density values, find the average density for each metal.
Average = Compare the theoretical density to the average experimental density by calculating the % error. x 100

Using your graphing calculator or LoggerPro, create a graph of mass (y-axis) versus volume (x-axis) for each metal. You will plot the five data points for each metal. Calculate the slope of each line. The slope represents the mass/volume or the density of the metal. Both lines can be plotted on the same graph so that the results can be easily compared.

Print out a copy of the graph to include in your lab report. Be sure to write the slope of each line on the graph.

Conclusions
1. State your results. What is the average experimental density for each metal?
2. State the theoretical value.
3. State the % error.
4. Think about and suggest at least two valid sources of error.
Suggest at least two ways to improve the experiment.

E Saylor

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