 Go ahead. We're trying to calculate here the average molecular weight of air. Of air. This is a very useful calculation that we would be needing in several of our problem solving things. We all know that air is composed of roughly nitrogen 79%. And O2 is 21%. We should remember that O2 is not 21 precisely, but we would use this approximate composition and try to figure out what is the molecular average molecular weight. We know that the molecular weight of nitrogen is 28, and oxygen is 32. Sorry, 32. So average molecular weight is actually mole fraction of each of these components, A times molecular weight of A plus mole fraction of B times molecular weight of B plus and so on. So in this case, we have only two components which is 79% or 0.79 times molecular weight of nitrogen is 28 plus 0.21 times molecular weight of oxygen, which basically comes out to be roughly 29. So average molecular weight is 29 for air. And if we do precise calculations, it might be slightly different. However, for the purposes of this class, if we take it as 29, that'll be good. Basically, what this means is 29 grams per gram mole. Or we can also say kilograms per kilogram mole. Or the same thing, 29 will not change pounds per pound mole.