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The mole and Avogadro's number

One mole of a substance is equal to 6.022 × 10²³ units of that substance (such as atoms, molecules, or ions). The number 6.022 × 10²³ is known as Avogadro's number or Avogadro's constant. The concept of the mole can be used to convert between mass and number of particles. Created by Sal Khan.

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Video transcript

- [Narrator] 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 gonna 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 it's average atomic mass is 6.94. 6.94 unified atomic mass units per atom, atom of lithium. What if there were a certain number of atoms of lithium such that if I have that number, so times certain number of atoms, then I will actually end up with 6.94 grams of lithium and this number of atoms is 6.02214076 times 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 masses 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, Amedeo Avogadro and in most contexts, because you're not normally dealing with data with this many significant digits, we will usually approximate it as 6.022 times 10 to the 23rd power. Now, there's another word that it's 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 Oswald, 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 of eggs, how many eggs do I have? Well, if I have a dozen of 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 6.02214076 times 10 to the 23rd lithium atoms. Exact same idea. It's just that Avagadro's Number is a much hairier of a number than a dozen.