The mole and Avogadro's number | Moles and molar mass | High school chemistry | Khan Academy

In a previous video, we introduced ourselves to the idea of average atomic mass, which we began to realize could be a very useful way of thinking about a mass at an atomic level or at a molecular level. But what we're going to do in this video is connect it to the masses that we might actually see in a chemistry lab.

You're very unlikely to just be dealing with one atom or just a few atoms or just a few molecules. You're more likely to deal with several grams of an actual substance. So how do we go from the masses at an atomic scale to the masses of samples that you see in an actual chemistry lab, or in, I guess you could say, our scale of the world?

Well, the chemistry community has come up with a useful tool. They've said, "All right, let's think about a given element." So say lithium. We know its average atomic mass is 6.94 unified atomic mass units per atom of lithium.

What if there were a certain number of atoms of lithium such that if I have that number, so times a certain number of atoms, then I will actually end up with 6.94 grams of lithium? This number of atoms is 6.022 476 * 10 to the 23rd power. So if you have a sample with this number of lithium atoms, that sample is going to have a mass of 6.94 grams.

Whatever its average atomic mass is in terms of unified atomic mass units, if you have that number of the atom, you will have a mass of that same number in terms of grams. Now, you might be saying, "Is there a name for this number?" And there is indeed a name, and it is called Avogadro's number, named in honor of the early 19th-century Italian chemist Amadeo Avogadro.

In most contexts, because you're not normally dealing with data with this many significant digits, we will usually approximate it as 6.022 * 10 to the 23rd power. Now, there's another word that is very useful to familiarize yourself with in chemistry, and that's the idea of a mole.

Now, what is a mole? It is not a little mark on your cheek. It is not a burrowing animal. Actually, it is both of those things, but in a chemistry context, a mole is just saying you have this much of something. The word mole was first used by the German chemist Wilhelm Ostwald at the end of the 19th century, and he came up with the word because of its relation to molecule.

Now, what does that mean? Well, think about the word dozen. If I say I've got a dozen eggs, how many eggs do I have? Well, if I have a dozen eggs, that means I have 12 eggs. So if I say I have a mole of lithium atoms, how many lithium atoms do I have? That means that I have 6.022 1476 times 10 to the 23rd lithium atoms.

Exact same idea. It's just that Avogadro's number is a much higher number than a dozen.