How to Calculate Density of a Gas
- 1). Measure or identify T, P, and V of the gas.
For example, if you pump up a bicycle tube that's 27 inches in diameter with a diameter of 1 1/2 inches, that's a volume of 150 cubic inches. Pump the tire up to 90 PSI, according to the pump's gauge. Assume the temperature hasn't changed and therefore is room temperature: 72 degrees Fahrenheit. - 2). Convert the terms to scientific units in order to use the more commonly known conversion factor in the ideal gas equation.
Seventy-two degrees Fahrenheit translates to 295 K (degrees Kelvin); 150 cubic inches translates to 2.458 L (liters); 90 PSI translates to 6.124 atm (atmospheres). - 3). Calculate the ratio n/V using the ideal gas law: n/V = P/RT, where R = 0.08206 amt-L / mol-K.
Continuing with the above example, n/V = 6.124/0.08206*295 = 0.2530 mol/L, where mol means "moles," a measure of molecule count. - 4). Convert the moles per volume to grams per volume.
The molar mass of air is 28.8 grams per mole. So 0.2530 mol/L converts to 7.29 grams per liter. This is the density of air in a bike tire at 90 PSI, or 6.1 atmospheres. That's the weight of about seven pennies in a tire.
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