Average molecular mass of the air

Average molecular mass of the air

Average molecular mass of the air is being computed in order to compare it with the molecular or atomic mass of compounds and elements and to see whether the gases are lighter or heavier than the air



For the first time the term the average molecular mass of the air I encountered in "Problems in General Chemistry" by Nikolai L. Glinka.
Average molecular mass of the air is understood as a weighted average of molecular and atomic weights of all gases of the air. For simplicity, an average molecular mass of air is calculated taking into account only nitrogen and oxygen as gases, of which the air consists. The average molecular weight of air taking into account the oxygen and nitrogen only is 29 grams in the volume of one mole of oxygen and nitrogen, i.e. 22.4 liters of a mixture of gases of oxygen and nitrogen with the same composition as in the air at normal conditions. What you can use average molar mass of air for? You can imagine a balloon filled with water vapor, and immediately state that the molar mass of water is 18 grams, which means that water vapor will move through the air upwards through the Earth's atmosphere, where because of lower temperature it will condense forming clouds.

Molar mass of water H2O 18.01528 g / mol
Molar mass of heavy water D2O 20.03 g / mol

In the Earth's atmosphere, water vapor of heavy water will condense lower above ground because of the difference of 2 grams between molecular mass between normal and heavy water. The isotopic separation between heavy and normal water will occur in clouds.
From the comparison of the molar mass of ordinary water H2O and heavy water D2O and HDO it shows that when it starts to rain, the first rain drops will be drops of heavy water rain from low clouds. Try to remind yourself at the beginning of any rainfall, when the first drops of rain were falling. The first drops of rain always hit louder the roofs of buildings, louder hit leaves on trees and louder fall down to the ground. After the first drops the rain no longer falls so loud.