Lesson

Background

An old riddle asks “Which is heavier, a pound of feathers or a pound of lead?” Of course, a pound of feathers and a pound of lead both weight the same. Nevertheless, there is clearly something different about a small lead brick and a large bag of feathers, even though they weigh the same. The key is to understand the relationship that exists between a substance’s mass and the volume it occupies. This relationship is expressed by the physical property called density. The density of a substance, unlike its mass or volume, is characteristic of the nature of the substance. A substance like gold has a particular density, which is different from that of iron or water or lead. Density is defined as the ratio of a substance’s mass to the volume it occupies.

Purpose

In this experiment you will accomplish the following:

• Determine the density of several liquids and solids.
• Identify an unknown metal by calculating its density from mass and volume measurements.
• Calculate the percent error within the class for a specific sample.

Procedure

Part I: Finding the density of liquids

1. Determine the mass of a clean, dry, empty 25-mL graduated cylinder using the balance.
2. Construct your own data table in the space below to record your data.
3. Pour 10 mL of water into the cylinder (make sure you’re looking at the bottom of the meniscus when measuring).
4. Measure the mass of the cylinder and water.
5. Calculate the mass of water.
6. Using the equation above, calculate the density of water.
7. Repeat steps one-five for samples of Pepsi and Diet Pepsi (no drinking allowed!)

Part II: Identifying an unknown metal by determining its density

1. Select one metal sample and one plastic sample to investigate.
2. Make an organized data table(s) in the space below.
3. Determine the mass of the metal to the nearest 0.001 grams using a balance.
4. Find the volume of the metal sample by water displacement:

• Fill a 25-mL graduated cylinder about half-full with water, measure and record the volume as accurately as possible.
• Carefully release the metal sample into the graduated cylinder so it is completely submerged in the water.
• The amount of water displaced is equal to the volume of the solid.

5. Calculate and record the density using the values of mass and volume.
6. Repeat this process for the plastic sample.
7. From a table of densities, identify the metal that you were working with.

Part III: Calculating percent error of an M&M

1. Obtain one chocolate M&M for your group (Do not eat it.)
2. Create a data table in the space below.
3. Determine the mass of the M&M using a balance.
4. Using the water displacement method determine the volume of the M&M.
5. Calculate the density of the M&M.
6. Write the density that you obtain on the board.
7. Once the accepted value is given, calculate the percent error.

Analysis

1. The density of water is 1.00 g/mL. How accurate was your calculation in Part I? What are some possible sources of error that could have occurred to result in an incorrect density value?
2. How confident are you with your calculations for the density of Pepsi and Diet Pepsi? What evidence do you have to justify your answers as being correct or incorrect?
3. Obtain the results for the density of an M&M from all other lab groups in the class (these should be written on the board). Were these results precise, accurate or both? Explain.
4. Would the water displacement method work for calculating the density of a tablespoon of salt? Explain.