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.

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.

#### Tip

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.