 In this lesson, we are going to talk about the most important quantities in chemistry that we will never be able to escape when studying this subject. They are mole, molecular mass and molar mass. So firstly, what is a mole? A mole is just another unit of measurement for an amount of substance, like grams or meters, but is used for particles. Particles can be atoms, molecules, ions or photons. Now since the last redefinitions in 2018, having a mole of particles now means that you have exactly, an emphasize on exactly, 6.02214076 times 10 to the power of 23 of those particles. Now the number of moles symbolizes lowercase n, the same way mass symbolizes lowercase n, and the unit for the number of moles is mole or moles for plural, but you can just write mole, mol, which is the symbol for the unit. The same way g is the symbol for grams or grams. Now 6.02214076 times 10 to the power of 23 is a very very big number, and that is precisely why mole was invented. It is because particles, such as those mentions, atoms, molecules, ions, and photons are so small that even just a few milligrams of them can contain so many particles that it is just tiresome and inconvenient to write down or even just to wrap our head around. So you can also use mole to describe things beyond particles. You can use it to describe pretty much any discrete entity, such as people, but it's a bit silly to do so because you would never likely do with 10 to the 23 people. Now let's talk about where that number came from. That number is actually called the Avogadro's number, and it symbolizes Na, capital N subscript capital A. We write Na equal to that number, so a unit invented for convenience should be defined for convenience. But before we talk about this, let's talk about molecular mass and molar mass.