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 mL3 so the units for density are g/mL3

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 y-intercept. 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 post-1982 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 0.90-1.39 g/mL ( see table 1).

Table 1: Densities of common polymers

Polymer

Density in g/mL

Common Use

Polyethylene terephthalate, PETE

1.38-1.39

Soda bottles

High Density Polyethylene, HDPE

0.95-0.97

Milk bottles

Polyvinyl Chloride, PVC

1.16-1.35

Shampoo bottles

Low-Density Polyethylene, LDPE

0.92-0.94

Trash bags

Polypropylene, PP

0.90-0.91

Bottle lids

Polystyrene, PS

1.05-1.07

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 well-organized 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 post-1982 pennies. 






Last edited by C. Stessman 2/16/05