 
Density Measurements
Purpose:
There are four parts to this experiment.
1,2  To measure the density of water by 2 methods.
3  To measure the density of pennies and estimate their elemental composition.
4  To measure the relative density of plastic (polymer)
samples.
Background:
Density is defined as mass per unit volume
d=m/v
where d is the density of the sample, m is the mass of thee
sample and v is the volume or amount of space occupied by the sample. Thus
to measure density we need to determine two values for a sample, the mass and
the volume. Usually the mass of an object is measured in units of grams
and the volume of a sample is measure in cubic milliliters or mL^{3} so
the units for density are g/mL^{3}.
Density and floating: In general objects placed in a fluid
that are less dense than the fluid float and objects that are more dense then
the fluid sink. The accepted value for the density of pure water is 1.00
g/mL and the density of ocean water at the surface is 1.03 g/mL. Objects
that float in ocean thus have a density of less than 1.03 g/mL.
Procedure:
1. Density of Water Choose a 30 or 50 mL beaker from your drawer. Determine the mass of the empty, dry beaker and record the value in your notebook, in a clearly labeled Table, with the correct number of significant figures. Add water to the beaker, filling it to a calibration line. The bottom of the meniscus, the curved water surface, should be at the line. Record the volume of the water. Determine the mass of the filled beaker and record the value. As always, use the number of significant figures appropriate to the measuring device. Repeat these measurements at least two more times.
2. A Better Value for the Density of Water The density of water can be determined graphically. The equation for density can be written as:
m = dV (1)
This can be compared to the equation of a straight line
y = mx + b (2)
where m is the slope and b is the yintercept. Comparison of equations (1) and (2) shows that if we plot the mass of a liquid vs. its volume, we should obtain a straight line which passes through the origin, and which has a slope equal to the liquid density. In this experiment you will determine the mass of several different volumes of water, and then graph the mass versus the volume. The slope of the best line through the points will be equal to the density.
Weigh a clean, dry 100 ml graduated cylinder. This and subsequent weighings should be to ±0.01 g. Do not handle the cylinder with your bare hands since oils and moisture from your skin will affect the mass. Handle it with a paper towel. Fill the cylinder as closely as you can to 10 ml with deionized water, read and record, in your Table, the volume of the water, estimating the last significant figure, measured to the bottom of the meniscus, as appropriate. Weigh the cylinder with the water and record your result. Repeat the procedure for volumes of about 25, 75 and 95 ml, always reading the exact volume, as above. After you have completed these mass and volume measurements, insert a thermometer in the water and determine the water temperature.
Repeat the entire procedure twice more.
3. The Density of Pennies: Obtain ten post1982 pennies. Determine their density. You may design your own specific method, but however you do it, you must describe your method in the Procedure section of your notebook. Please return the pennies when you are finished.
4. Different types of plastics, also called polymers, have
different densities ranging from 0.901.39 g/mL ( see table 1).
Table 1: Densities of common polymers
Polymer

Density in g/mL

Common
Use 
Polyethylene terephthalate, PETE

1.381.39

Soda bottles 
High Density Polyethylene, HDPE

0.950.97

Milk bottles 
Polyvinyl Chloride, PVC

1.161.35

Shampoo bottles 
LowDensity Polyethylene, LDPE

0.920.94

Trash bags 
Polypropylene, PP

0.900.91

Bottle lids 
Polystyrene, PS

1.051.07

Egg cartons 
Pure water has a density of 1.000 g/mL, however, other
liquids have different densities. (table 2) You will use these known densities
of liquids to help identify polymer samples.
Table 2: Densities of various liquids
Liquid 
Density in g/mL 
Distilled water 
1.000 
70% isopropanol / 30% water 
0.930 
corn oil 
0.917 
Fill three 50 mL beakers with 25 mL each with one of the
following solutions, distilled water, 70% isopropanol/30% water, and corn
oil.
Obtain three unknown polymer samples and
determine their relative densities. You do this by seeing if each sample
floats or sinks in each of the three solutions. Clean off your polymer
samples with a paper towel between each solution. You are not able to
determine the exact density of the polymer sample just its relative range.
Using this range come up with possibilities for the identity of your polymer
samples.
Note: Cleaning up after corn oil. You may need to use the
lab soap and water to clean up your beaker containing the corn oil.
Do not discard the polymer samples. Dry and return!
Results:
In every part of this section, you must provide sample calculations, written in your notebook,
including all units.
Part 1: Density of Water
Part 2: A Better Value for the Density of Water
Make tables of your data and calculate the slopes. Calculate the average density from the slopes
of your measured densities.
Part 3: The Density of Pennies
Calculate the density of the pennies from the data for each trial, and the average density
of the measured densities.
Part 4:
Calculate the relative density of each of your three polymer
sample, using table 1 come up with potential identities of your
samples.
Conclusions:
Summarize your key results in a wellorganized format. Be sure to include the following:
The values for the density of water from each method. Compare the two valuess. Are the results reasonable? Why?
The relative densities and potential identities of your polymer unknowns.
The average value for the density of post1982 pennies.
Last edited by C. Stessman 2/16/05
  