 So, today we're going to talk about the mole, and mole is a way of measuring atoms and molecules. And so, when you think about a mole, it might be useful to think about some common other ways that we measure things like in the grocery store. So, if you have sugar in the grocery store, you're going to measure that by the pound. So, you might buy a five pound bag of sugar. If you've got milk in the grocery store, you might have a gallon. And if you've got eggs in the grocery store, you're going to have a dozen. And so, this is the most useful one for thinking about a mole. Because a dozen is a number. It's twelve. It's just another way of saying the number twelve is to say one dozen. So, a mole is just another way of saying another number. That number is called Avogadra's number, and it's written here in two different ways. It's written in the long hand version, which is really, really long and a very large number. So, we've converted that using scientific notation to 6.023 times 10 to the 23rd. A mole is used, that number 6.023 times 10 to the 23rd is used to measure atoms and molecules. So, one mole of hydrogen atoms, for example, is equal to 6.023 times 10 to the 23rd hydrogen atoms. And so, if you are measuring out a mole of hydrogen atoms, you could count those hydrogen atoms if you could count them. You would come up with exactly 6.023 times 10 to the 23rd. It would take you forever to count them, and they're far too small for you to count. But luckily, there's a good way to know exactly how many atoms are in a certain sample of hydrogen. So, let's take a look at our periodic table, and we'll look at hydrogen. So, hydrogen has an atomic number of 1, and it has atomic mass of 1.00, let's just call this 1.01. So, hydrogen has an atomic mass of 1.01. So, what does that mean in terms of our moles? Let's write this down. And that's 1.01 grams per mole. So, if we want to have one mole of hydrogen atoms, we want to have 6.023 times 10 to the 23rd hydrogen atoms, we would need to measure out 1.01 grams of hydrogen, and we would know that that contains 6.023 times 10 to the 23rd hydrogen atoms. It works similarly with molecules. So, let's take a look just at table salt, and we'll just figure out how many grams of salt would have one mole of molecules. So, again, let's look at our periodic table. We're done with hydrogen, so we're going to need sodium, which the atomic mass of sodium is 22.99. So, we'll go back and write that down. Sodium 22.99. And chlorine, back to the periodic table, 35.45. So, the atomic mass of this molecule, sodium chloride, is 22.99 plus 35.45. We have 58.44 grams per mole. You should always be labeling these grams. And I didn't mention this before, but the abbreviation for mole is really not all that abbreviated mole MOL. So, if you see MOL, that's mole. So, we know that in 58.44 grams of salt, we have one mole of sodium chloride molecules. So, if we counted the number of sodium chloride molecules in 58.44 grams, we would come up with 6.023 times 10 to the 23rd. People may get a little confused between moles and molarity, so we're going to discuss molarity at another time. But just be aware that moles are not the same as molarity. Those are two different measurements.