 Okay, as promised, a slightly new topic. We're going to talk about molar mass, but we're going to extend the idea of molar mass. A couple of videos back, I told you that molar mass is basically answering the question, how much does, cannot spell to save my life apparently, does one mole of something way. And I put way in quotes because I really should be saying mass. So more formally molar mass is what's the mass of one mole of something, but we're just going to say how much does one mole of something weigh. And I said, you know, you can use the periodic table to figure this out. You just look up the number in the box for your particular element and you tell me in grams. So I think, you know, carbon, 12.011, something like that, and 6 for the atomic number. This is the average atomic mass, so if I said what's the molar mass of carbon, you'd say it's 12.011 grams, or 12.011 grams per mole. Either way is okay. We're going to extend this idea. So here we go. What's the molar mass of one mole of hydrogen? Well, it's one gram, roughly. Let's just round to the nearest little number. What's the molar mass of one mole of oxygen atoms? It's about 16 grams. What's the molar mass of one mole of water? Water molecules. What you do to answer this is you say, look, one mole of water has two moles of H atoms. Because every water molecule that you crack open outtumbles two hydrogens. So if I have one mole of water, I must have two moles of hydrogens. And every water molecule that I crack open outtumbles one oxygen atom. So if I have one mole of water, I also have one mole of O atoms. But you just told me that one mole of hydrogen weighs one gram. So two moles of hydrogen must weigh two grams. And so inside of every one mole of water is two grams of hydrogen. Inside every one mole of water is 16 grams of oxygen. Because you told me one mole of oxygen weighs 16 grams. So inside of every one mole of water is two grams of hydrogen and 16 grams of oxygen. So one mole of water weighs 18 grams because we add up the weights of all of the pieces. So what we just did was we figured out the molar mass of something more complicated than a simple atom. We figured out the molar mass of a molecule. In this case it was water. And the molar mass of water is 18 grams or 18 grams per mole. And the way that we did it is we figured out the weight of all of the pieces in one mole of water. One mole of water has two moles of hydrogen in it. Two moles of hydrogen weighs two grams. One mole of water has one mole of oxygen in it that weighs 16 grams. All together all of the pieces weigh 18 grams. So the molar mass of water is 18 grams or 18 grams per mole. You can do this now. You can do this for any molecule that you know the formula for. You're not just stuck with figuring out the molar mass of water. Something that I give you the formula for, you add up the mass of all of the pieces of the elements in the formula and then you tell me how much it weighs in grams. How much one mole weighs in grams. So here's sort of a summary of how to figure out the molar mass of a molecule. As long as you know it's formula you can figure it out. You look up the molar mass of each element in your molecule. If we were doing water we'd say, you know, we want it to be fancy-pancy. The molar mass of hydrogen is 1.0079 grams. Molar mass of oxygen is 15.999 grams. You multiply each molar mass by the number of atoms in each number of atoms in your molecule. So we have to multiply this times two because there's two hydrogens in the formula for water. And we have to multiply this times one because there's only one oxygen. So if you do that and then you add the numbers together it tells you how much one mole of your molecule weighs. If we did it more carefully that's basically the molar mass of one mole of water. It's 18.014 grams. So as far as we're concerned it's about 18 but that's how to do it. You can do this for any molecule where you know the molecular formula. So here you go. Here's a test. What's the molar mass of carbon dioxide? This is the formula for carbon dioxide. Here's some relevant information. You can pause the video and work through this on pausing. Well we have one carbon and we have two oxygens. So one mole of carbon weighs about 12.011 grams. Let's just round to 12 grams. One mole of oxygen weighs about 16 grams but we have two of them. So we have to multiply this number times two. That's 32 grams. 12 grams plus 32 grams is 44 grams. So molar mass of CO2 which is also known as carbon dioxide is about 44 grams per mole. So if you had a balloon filled with 44 grams of carbon dioxide you would have one mole of CO2. Or put another way, if you had 44 grams of carbon dioxide in a balloon in that balloon there would be about 6.02 times 10 to the 23 CO2 molecules bouncing around. And so all these videos have just been 26,000 different ways of showing you how to convert between how much something weighs and how many molecules you have and every once in a while stopping along the way to convert to moles or vice versa, telling you how many molecules you have, converting that into groups of 6.02 times 10 to the 23 which is also known as counting moles and then converting that into weight. So either direction you should be able to do. I know it's not the easiest thing but that's why you're getting paid the big bucks or something like that. That doesn't make any sense, I don't expect it to make sense. It's a bad joke, can't all be a material. Okay, last question in this enormous unit. You have 100 grams of NACL, this is also known as table salt. How many moles of table salt do you have? This is a little bit more difficult, here's some relevant information. You can pause the video and try and answer this, might be a long pause. Anyway, I'm pausing, what you have to do is you have to convert this to moles, right? I'm telling you how many grams, you have to convert that to moles. But what you have to do before that is you have to figure out how much a single mole, how much does one mole of NACL weigh? Really this is just saying what's the molar mass of NACL. If we can figure out what the molar mass of NACL is, then we can figure out how many moles are in 100 grams. So to figure out the molar mass of NACL we have to add up the molar mass of the pieces of sodium and chlorine together. If you add up these numbers, I think I've done this enough, I think I've done this enough that I'm pretty sure it comes out to about 58.4 grams of NACL is one mole of NACL. So here I'm rounding to the tenths place, it's okay if you round it to the ones place, that's perfectly fine. So the molar mass of NACL is about 58.4 grams per mole and I'm writing this as a ratio again. But that's how much one mole weighs, one mole weighs 58.4 grams, but I don't have 58.4 grams in my question, I have 100 grams. So I'm going to set up an equal fraction off to the right and instead of 58.4 grams I'm going to write 100 grams of NACL and I'm going to say well I want to know how many moles that is, X moles of NACL. Now we can just go to town and cross multiply to figure out what X is. Before we do that however, we might as well conduct a sanity test. Is this number 100 bigger than 58.4? You know what it is, so should this number X be bigger or smaller than one? It should be proportionately bigger. So we better come up with a number that's bigger than one. Not only that, we can be a little bit fancier, we can say well 100 is almost twice as big as 58.4 because 50 plus 50 is 100, 58.4 plus 58.4 is going to be a little bit bigger than 100, but this number is almost twice as big. So whatever X is, it better be almost twice as big as one. So it better be almost two, but not quite. So it should be a little bit under two, but close to it. So let's cross multiply. We do one times 100, so we do one mole of NACL times 100 grams of NACL is equal to these other numbers multiplied together, 58.4 grams of NACL times X moles of NACL. We want to get the X all alone, we want to get rid of this guy because it's tied to the X at the moment. So we divide both sides by 58.4 grams of NACL, to do the same thing to the left side, 58.4 grams of NACL divided by grams of NACL cancel, that unit goes away, 58.4 divided by 58.4 reduces down to one. So now we have the X all alone, grams of NACL, grams of NACL those units go away. The only unit I'm left with on the left is moles of NACL. The only unit I'm left with on the right is moles of NACL, which is what I wanted. So one times 100 times 100 divided by 58.4 is equal to X moles of NACL. And what does that turn out to be? It's about 1.71 moles of NACL. Did we go insane? Well, our number was supposed to be almost two, but not quite. 1.7 sounds like it's in that neighborhood. So we probably did the calculation correctly. So if you have one mole of NACL, it weighs 58.4 grams. And we got that information by looking up some numbers in the periodic table. If we have 100 grams of NACL, we can use this information to figure out that 100 grams of NACL is 1.71 moles of NACL. And so that's how you convert grams to moles for more complicated molecules. For more complicated molecules, you have to add up the molar masses of all of the pieces in your molecule and then use that information. But then once you do that, the calculations are the same as what we've done before. So that's it for this unit. We've covered molecular formulas. Let's probably say molecular. We've covered counting atoms, introducing the mole and Avogadro's number. We've covered weighing atoms. So how much does one mole of atoms weigh? And that's the idea of molar mass. We've also covered converting between number of molecules, number of moles, and weight of molecules. And finally, we just finished talking about molar mass of more complicated molecules instead of just atoms. So that's it. Good luck on your test or whatever you're doing this week. Bye-bye. Whatever. I went down to the beach and saw Kiki. She was all like, eh, and I'm like, whatever. And this chick comes up to me and she's all like, hey, aren't you that dude? And I'm like, yeah, whatever. So later, I'm at the pool hall and this girl comes up and she's all like, eh, and I'm like, yeah, whatever.