VIDEO: How to calculate the volume of air

The molar mass of air

In order to calculate what volume a certain amount of air has, you first have to find out what molar mass it has. The molar mass indicates how many grams 6,022 * 10²³ Weighing particles of a certain substance. One calculates with the mean atomic mass of all elements involved.

1. The first step in your calculation is to study the air composition. When you do this, you will find that air is about 78% nitrogen (N.₂), 21% from oxygen (O₂) and about one percent consists of other gases.
2. Now you have to insert their molar masses into the percentage distribution of the gases. With nitrogen this is 28 g / mol, with oxygen 32 g / mol. For the sake of simplicity, you should leave out the other gases.
3. If you multiply the molar masses by the percentage, add the results, and divide the final result by 100, you will know how many grams a mole of air weighs. Incidentally, the result is 28.56 g / mol. Of course, this is only an approximate value.

How to calculate the volume of air

Before you start working out the volume of air, you need to realize that it is not a constant. Air pressure and temperature influence the volume directly and therefore do not allow any general statements.

• You can now calculate the density of air as follows: p (air pressure) * M (molar mass in kg / mol) / R (universal gas constant) * T (temperature in Kelvin). Under normal conditions (atmospheric air pressure p₀= 1013.25 hPa and T₀= 273.15 K (which corresponds to 0 ° C) the air density is 1.293 kg / m³.
• In order to calculate the volume of a certain amount of air, you must now divide the mass of the amount of air in question by the previously calculated density. The volume here is in m³ before.
• For example, the calculation for 100 kg of air (at p₀= 1013.25 hPa and T₀= 273.15 K): V = 100 kg / (1.293 kg / m³). The result is 77.3 m³. With the given values ​​for pressure and temperature, 100 kg of air have a volume of 77.3 m³.