 So in last lessons, we learned about the atomic mass or average atomic mass, which is the average mass of an atom of a specific element. So what about the average mass of a molecule? So-called molecular mass. So the molecular mass of a molecule is simply defined as the sum of the average atomic mass of individual atoms that make up the molecule. Let's do a question. Let's calculate the molecular mass of a carbon dioxide molecule. So carbon dioxide consists of one carbon atom and two oxygen atoms. So the molecular mass of carbon dioxide, so it's m02, from the periodic table, the average atomic mass of carbon equal 12.011 Dorton, and the average atomic mass of oxygen is equal to 15.999 Dorton. So the molecular mass of carbon dioxide, c02, is equal to 40.021 Dorton. So that's the molecular mass. What about the molar mass? Molar mass is actually very simply just the mass of a mole of an amount of substance. So say you have one mole of water, then the molar mass of water is simply the mass that in a water molecule's weight. So the molar mass is actually about 18.01528 gram per mole. So the unit for molar mass is grams per mole. The symbol for the quantity molar mass is capital M. So how do we calculate this number? Now if you try to calculate the molecular mass of water, you will realize that water's molecular mass is equal to 18.015 Dortons, which is a similar number to its molar mass. And this is precisely how the Avogadro's numbers were designed. The value of the Avogadro's number was chosen so that the mass of one mole of a chemical compound in grams is numerically equal to the average mass of one mole of the compound in Dortons. So when you want to calculate the molar mass of any compounds, just calculate the molecular mass of the compound and then assume that the molar mass will have the same value.