The molecular weight (molar mass) of a substance is the mass of one mole of the substance, which can be calculated based on the molar mass of the constituent atoms. Dry air is composed of two important chemical elements: 78% nitrogen and 21% oxygen, and about 1% are mixtures of carbon dioxide, neon, helium, methane, krypton, hydrogen, and xenon.
Before further calculating the dry air molar mass, we must understand the notion of the mole. Mol is a unit of measurement for the number of substances, which denotes the number of representative particles equal to the number of atoms in 12 grams of carbon-12 atoms (12 C). The number of particles is expressed in Avogadro Numbers which is equivalent to 6.022140857 × 10 23 particles / mol. One thing to note is that one mole of nitrogen, has a size of the number of constituent atoms equal to one mole of oxygen, one mole of carbon dioxide, and one mole of other substances present in the world.
Now let’s calculate how much dry air molar mass goes through the table below:
The molar masses of each of the above air constituent components are calculated according to the standard data from the periodic table of chemical elements. Nitrogen for example, with the chemical formula N2, has an atomic molar mass of 14.007 g/mol. Then the molar mass of 2 nitrogen atoms (which make up N2) is 28.014 g/mol. The same calculation is also performed for other dry air constituent elements.
Furthermore, the molar mass of each constituent element is multiplied by the percentage value of the element content in the dry air. From this we get the elemental element molar value. After the elemental molar mass of the portion is summed, we obtain a molar mass for air of 28.9647 gram/mol.