 Okay, I want to start this unit by reviewing what was covered in the old unit because moles and things related to moles are often difficult for many, many of the students. So we're going to recover a little bit of what the mole is. One mole of something is just an easier way of saying one group of this many things. Now it's almost always reserved for talking about atoms and molecules because that's about how many you need to have before you can actually start to see groups of atoms and molecules and weigh them out. One mole of an atom, a particular atom, you can figure out how much it weighs by looking at the average atomic mass unit and changing the unit to grams, and I'll try to explain that in a couple of slides as well. And finally, this number is sometimes known as Avogadra's number or Avogadra's constant. The points on this slide get their own special slide because they're important enough that they're going to show up over and over again. So I have two big bullet points here. If you know the number of moles of something you have and you know it's chemical formula, you can figure out how much it weighs. You can figure out how many grams of it you have, and you can do this in reverse. If you know how many grams of something you have, in other words if you know how much something weighs and you know it's formula, you can figure out how many moles of it you have. So you need the periodic table to do this. Here's information on chlorine from the periodic table. If I told you that I had one mole of chlorine, you could look at the periodic table and you could say, oh, you have 35 grams, roughly. If I told you you had two moles of chlorine atoms, you could look at the periodic table and say two times this number, which is about 71 grams, you could say, oh, you have 71 grams of chlorine. In other words, what we're doing is the first bullet point here. If you know the number of moles and you know the information about its molar mass, you can figure out how much it weighs. One mole weighs 35 grams, two moles weighs 71 grams, and you can do this in reverse. It's not just chlorine that you can do this for. You can do it for any element where you know where you have the periodic table handy. If I told you that I had 35.453 grams of chlorine, in other words, if I told you how many grams you have, you can use the periodic table to figure out that that's one mole of chlorine. So you should be able to go in either direction, moles to grams or grams back to moles. Then the other major term that we introduced in the previous unit is called molar mass. Molar mass is mass or weight of one mole of something. How much does one mole of something weigh? Molar mass is basically answering this question. How many grams does my pile of one mole of atoms weigh? I will try to explain that in a little bit more detail on the next slide as well. So here's the next slide. What's the difference between a mole and molar mass? A mole is counting. It's how many groups of 6.02 times 10 to the 23 do we have? So I want you to think of this as a pile of atoms, a mountain of atoms. And in my mountain, apparently the pen doesn't want to work that well right now, in my mountain there are 6.02 times 10 to the 23 atoms and each circle there is supposed to be one atom. So there's a lot of atoms there, one mountain's worth or one mole's worth. That's equal to 6.02 times 10 to the 23. So if you could pick apart this mountain's worth of atoms and count each one, you would count this many. That's basically, so if I said I had one mountain's worth of atoms, in my analogy I would say I'd have one mole's worth of atoms. If I had two mountains that were identical in number, then I would have two moles. And so that's what a mole is. It's just saying how many groups of mountains do you have? Or how many groups of 6.02 times 10 to the 23 do you have? That's what a mole is. Molar mass, as we mentioned on the previous slide, is how much does this mountain actually weigh? There's a big difference, right? This is a counting question. This is a weighing question. It's weighing by each mole or, to use my analogy, each mountain. So molar mass is not counting. It's saying, look, if I have one pile, one giant pile of 6.02 times 10 to the 23, or one mountain's worth of atoms, how much does this pile actually weigh? And the unit is almost always given in grams. So the molar mass is answering this question. How much does one mole of something weigh? So as an example, the way that you answer this particular question is you have to go back to the periodic table and you have to know what type of atom you're talking about. If I told you that I had one pile, one mountain's worth or one mole's worth of boron atoms, here they are. And I wanted to know how much this pile weighs. You go to the periodic table, you look at this number, and you tell me this number in units of grams. So that's telling you, if these were all boron atoms, and I had this many of them, it happens to weigh about 10.811 grams. And the molar mass is going to be different for different atoms, because different atoms weigh differently than each other. If I had a pile of carbon atoms, a mountain of carbon atoms, a mole of carbon atoms, if you want, and I wanted to know what its molar mass was, I am asking how much does this pile weigh? As you can see, it weighs more than the pile on the left, because the individual atoms weigh more. You can look that up here. If I said what's the molar mass of carbon, you would say it's about 12.011 grams. And again, you can see this visually. So if I have one mole of boron atoms and one mole of carbon atoms, I have the same number of each. There's 6.02 times 10 to the 23 borons here. There's 6.02 times 10 to the 23 carbons here. But the molar masses are different. In other words, this pile weighs less than the pile of carbons. And you can look up specifically how much each weighs if you have a good periodic table handy. If I showed you a mole of nitrogen atoms, pretend this is a mole of nitrogen atoms, I have the same number of nitrogens as I do carbons as I do borons. But the nitrogens weigh even more. And so if you weighed this pile, you would be telling me what the molar mass of nitrogen is. And if you had a periodic table, you'd say, oh, it's about 14.007 grams. So that's the difference between a mole and a molar mass. Mole is a way of counting. It's a weird way of counting, but that's all it is. Molar mass is a way of weighing. That weighs by these piles. How much does one pile of nitrogen atoms weigh? How much does one pile of carbon atoms weigh, et cetera? So that's it for the review. New stuff coming up.