To calculate the density of any material, you must learn that density is the amount of matter–mass–contained within a fixed amount of space–volume. Mathematically, you can express density as mass divided by volume. For units, use grams (g) for mass, and cubic centimeters (cm^3) for volume. To determine the density of a sample of plastic, you simply determine its mass and volume, then divide the two numbers.
Obtain the piece of plastic for which you want to know the density. If the plastic object is large, it is safe to assume that the plastic is of uniform density throughout, so taking a small sample of the plastic for measurement will make things easier.
Use your balance to determine the mass of your piece of plastic. Record your sample's mass in grams. If you must use a scale that measures in pounds, weigh the plastic, then multiply the weight in pounds by 454.5 to convert to grams.
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Example: 2 pounds x 454.5 = 909 grams
Fill your graduated cylinder to the 500 mL mark with water. Carefully lower the piece of plastic into the water until it is fully submerged. Record how much the water level rises; this is the volume of your plastic in cm^3.
Example: If the water rises to the 625 mL mark, then it rose a total of 625 - 500 = 125 mL.
1 mL of water = 1 cm^3, so the volume of your piece of plastic is 125 cm^3
Divide the mass value by the volume value to determine the density of the plastic.
Example: Density = mass/volume = 909 g / 125 cm^3 = 7.272 g/cm^3
The density of your plastic in this case is 7.272 g/cm^3.
Be careful not to trap air inside your piece of plastic when you lower it into the water. This would result in an inaccurate volume reading.