 In today's video, we're going to talk about this unit called moles. All right, so what is a mole? A mole is simply a unit of measurement representing a number of items. So when you go to the grocery store and you buy a dozen eggs, you expect every time you're going to get 12 eggs. The word dozen is a unit of measurement for a particular number. Same thing for moles. Moles is a unit of measurement representing an exact number of particles. Another definition for the term moles is a group of atoms, molecules, or formula units that contain 6.02 times 10 to the 23rd power of these atoms, molecules, or formula units. So this number here, the 6.02 times 10 to the 23rd power, this is the important value in this term of moles. All right, this value is called Avogadro's number. Just like a dozen means 12, a mole means 6.02 times 10 to the 23rd power. So if I were to say I have one mole of carbon atoms, I'm simply saying that I have 6.02 times 10 to the 23rd carbon atoms. All right, how do we use it in terms of a molecule? First of all, what is a molecule? A molecule is made up of two non-metals, two or more non-metals, bound together. They form a covalent bond, something similar to carbon dioxide. Carbon dioxide would be a molecule. If I have one mole of carbon dioxide, then I simply have 6.02 times 10 to the 23rd carbon dioxide molecules. The same thing goes for formula units. Formula units just refers to ionic compounds. Ionic compounds come from metals bound to non-metals. An example of a formula unit would be potassium sulfate. Potassium sulfate is made up of a metal potassium found on the left side of the periodic table bound to sulfate, which is known as a polyatomic ion. But in the same way that one mole equals 6.02 times 10 to the 23rd carbon atoms, if I have one mole of a formula unit like potassium sulfate, then I have 6.02 times 10 to the 23rd potassium sulfate formula units. Now, how do we use this mole term in a problem? So here we have a problem that's asking how many iron atoms are in four moles of iron? So in order to solve this problem, we first have to recognize that this is a conversion problem. This problem is asking us to convert four moles of iron to how many molecules of iron we have. Anytime we are presented with a conversion problem, you're always going to start out with determining an equality which relates your unit that you're given to the unit that you want. It turns out we have that equality. One mole is equal to 6.02 times 10 to the 23rd atoms. This is an equality. This is stating two equal values. From all the qualities, we get two conversion factors. A conversion factor is simply stating an equality in fractional form. So our two conversion factors here is going to be one mole over 6.02 times 10 to the 23rd iron atoms. And our second conversion factor is 6.02 times 10 to the 23rd iron atoms is equal to one mole of iron. Now we're ready to set up our conversion. In order to set up your conversion, you first can determine what value is the problem giving you that it wants you to convert to a different unit. In this case, it's giving us four moles of iron. Now we need to pick a conversion factor that will allow us to convert from moles to number of atoms of iron. So we have either of these conversion factors here to choose from. The one that we're going to choose is this second one. Now why do we choose this particular conversion factor over this one? It all has to do with making sure our units can't sell out. Whenever we multiply 6.02 times 10 to the 23rd iron atoms over one mole of iron, what you see is going to happen is this unit of moles of iron and this unit of moles of iron will cancel out. What's the only unit we're left with? It's the number of atoms of iron. So we multiply 4 times 6.02 times 10 to the 23rd and then we divide by 1 and we find that 4 moles of iron is equal to 2.41 times 10 to the 24th power. This is the number of actual iron atoms that are contained in 4 moles of iron.