When we look at a glass with no water in it or a paint can after all the paint is used, we usually think of it as empty. However, these cylinders aren’t really empty. They are full of gas: the air that surrounds us. Air, as well as gases such as hydrogen and helium, has mass. If you could put a gas on a scale, you’d find it has a specific weight that depends on the density of the particular kind of gas. However, you can figure the weight of gas in a cylinder if you calculate the volume of the cylinder and know the density of the gas it contains.

The standard densities of gases listed in tables assume that the atmospheric pressure is normal (about 14.7 pounds per square inch) and that the temperature is about 60 degrees F (15.6 degrees C). If the air pressure is lower, the gas in the cylinder will weigh less. The same is true if the temperature is higher than 60 degrees F because gas expands as temperature rises. Conversely, higher pressures and lower temperatures result in more air in the cylinder.

Find the radius of the cylinder. Wrap a tape measure around the cylinder to measure its circumference. Divide this value by 2 pi (about 6.283) to find the radius of the cylinder. For example, if the cylinder is 26 cm in circumference, the radius is 26 cm/(2 pi), or about 4.12 cm.

Measure the height of the cylinder. To calculate the volume, use the formula V = H x pi x R^2 (volume equals height times pi times the square of the radius). Suppose you have a cylinder 10 cm high and the radius is 5 cm. You calculate V = 10 cm x 3.14 x (5 cm)^2, or about 785 cubic centimeters.

Find the density of the gas in a table of gas densities. Air has a density of 0.128 g per cubic centimeter (this is often listed per milliliter). Different gases have other densities. For example, helium has a density of 0.00018 g per cubic centimeter.

Multiply the volume by the density to figure the weight of gas in the cylinder. A cylinder with a volume of 785 cubic centimeters filled with air contains 785 x 0.128 g, or about 100.48 g of air.

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- Theresa Knott, Wikipedia Commons