In our ideal gas law, we don't have a variable to distinguish one gas from another. For example, one mole of helium, xenon, or methane gas would each occupy the same volume at the same temperature and pressure. The volume one mole of a substance occupies is called its molar volume. At standard temperature and pressure, or STP, 1 mole of helium, xenon, or methane gas would each occupy 22.4 liters. So how can we tell these gases apart? One physical property that is unique for these three gases is density. The density of a gas is its molar mass divided by its molar volume. Although all three gases have the same molar volume, they have different molar masses. We can calculate density as pressure times molar mass divided by the ideal gas law constant and temperature. Density will have units of grams per liter when the molar mass is in grams per mole. Let's practice using density to identify a gas. Our problem reads, "A 12.4 liter sample of a gas has a mass of 0.984 g at a temperature of 22.4°C and a pressure of 0.954 atmospheres. You know this sample came from one of five gas tanks in your lab: carbon dioxide, oxygen, nitrogen, helium, or hydrogen. Which gas should it be?" We'll start with our equation for density. We want to rearrange the equation to solve for molar mass. Density has units of grams per liter, so we can divide the mass of the gas by the volume and substitute it into our equation for density. We also need to be careful to convert degrees Celsius to Kelvin for temperature. Now our units of liters cancel out, atmospheres cancel out, and Kelvin cancels out; we're left with units of grams per mole. Completing the calculation, we should get 2.02 grams per mole. We can compare this to the molar masses of the gases in our five tanks. Notice that oxygen, nitrogen, and hydrogen gases are all diatomic. According to the molar mass, our gas should be hydrogen.