In this video, the important chemical concept called stoichiometry will be introduced. I'll begin with describing stoichiometry and how it relates to chemical formulas, or formulae with the letter e at the end instead of s, if you prefer the formal latin. Stoichiometry is a long word for determining relative quantities in chemistry. Really, it's just a matter of determining the correct mathematical ratio between atoms and a molecule or between two or more molecules. We'll take it one step at a time. And once you've mastered how to use stoichiometric ratios, you'll find that it's much easier to perform chemical calculations. There's several different types of stoichiometry. The tasks that you will learn how to master include determining the number of atoms in a molecule or the chemical formula, determining mass of a molecule, balancing chemical equations, and converting from mass to moles and back. There's all kinds of other calculations where one can use stoichiometry. And let's go ahead and start now with using stoichiometry to determine the number of atoms in a molecule. Believe it or not, you've been using stoichiometry your entire life. Let's use an analogy of a baskeball team. So a basketball team, here's the court that I've drawn, typically has different types of positions. I'm going to assign each player a type of position. I'm going to play with two guards, drawn here, two forwards down low post, and I'm going to put the center right in the middle, although they're not really allowed to stay there very long. Because of the rule, you can only stay in the cube for three seconds. So in my type of basketball team, I, I will always have two guards, two forwards, and one center. You see the center's a little taller than the other players. So I could actually write a formula for a basketball team. If my basketball team has five and only five players, the formula for one team will be CF2G2. What I've done is made a symbol for each position type. I've made the center the symbol C. The forwards have the symbol F. There's two of them, that's where that two comes from, and the guards has the symbol G and there's also two guards. There's a one center but the one is understood there, so we don't normally draw it in. So the formula for one team is CF2G2 and that's a way of showing the stoichiometry or the ratio of the different types of player positions. What if I have two basketball teams. If we're going to continue our basketball analogy for stoichiometry, in order to play a game, we need two teams. So to have two teams, I would put a two up front of the symbol for basketball team. Now when I have two teams, I need to have two centers. If I look at the types of positions. And you'd have four forwards. We could actually count up the players on the court. Here's my forwards, one, two, three, four. And how many guards would I need? I also would need four guards. Now, don't worry if you don't know anything about basketball, or you don't care about basketball, or you don't like sports. I'm just trying to show you the stoichiometry that we use in chemistry can be applied to other things. How many total players do I have on the court? Well, with two basketball teams of course, I can add up all of the players and see that if I add up all the individual components, I have ten players. The formula here is the key. It doesn't matter how many teams I have, I can quickly do the calculation for how many of each player, each type of position I need, if I have the formula. Let's do an example. How many centers and forwards are needed, in order to have six basketball teams? Well as I just said, the chemical formula is the key. Here is the basketball team formula not the chemical formula. The team formula is the key, so to build six teams, here's what we need. First let's ask ourselves, how many centers are needed for six teams? Some of you can just do this in your head, but here's what you're really doing, you're saying okay, I have six teams, the symbol for team member is CF2G2, and then you're using a ratio of the number of centers per team. There's one center per team, and the ratio can be written this way. One center, per one team. Now if I do the calculation, the symbol for the team canceled out, the units for the team cancel out. And that calculates to needing six centers on six teams, see how they cancel out there? You can do a similar calculation for the number of forwards you need to have six teams. Again, you start with the six teams, this time the ratio is different. Because, in the formula it tells you have two forwards per team, but the symbol for the team cancels out, and you're left with 12 forwards when you do the calculation, as the units. Remember, everything needs to have a number and a unit. That's really important. Now, let's start looking at groups of teams. How about an imaginary quantity? I'm going to make up an imaginary quantity, and I'm going to call it tournament. Now, you can imagine, a tournament can have any number of teams or any number of players, but I'm going to define the tournament as having eight of something, so I'm defining this quantity, the tournament, as the number eight. In other words, an octopus has a tournament of legs. You can say that, because tournament just means eight, the way I've defined it. Let's ask ourselves then; how many guards are there in a tournament of basketball teams? In order to do this calculation, I start with what I'm given, which is one tournament. Should've abbreviated tournament, shouldn't I? I'm going to multiply that by the number of teams in a tournament. And I'm going to use the formula for a team in my calculation. There's eight teams in one tournament. I'm just going to abbreviate it tourn because I'm lazy. And then the question is, how many forwards are in one tournament of basketball teams? So I need to look at the formula and realize that there are two forwards per team, so I can a ratio using the stoichiometry of the team. There's two forwards per one team. Now I can cancel the units. Do we need an analysis? Teams, tournaments cancel. Teams cancel. And I'm left with, there are 16 forwards in my tournament. You can do all kinds of crazy analogies using stoichiometry, for example, let's consider a Quidditch Team. Doesn't really matter if you know what a Qudditch team is, because I'm going to tell you. Some of you love Qudditch and maybe even play Qudditch in your spare time. But, on a Qudditch team, there are four different positions and every team during play has one keeper, two beaters, three chasers and one seeker. That's the best I can do with a British accent. I guess I'm not Benedict Cumberbatch or Alan Rickman. anyway, so let's go ahead and write the formula for a Quidditch Team. So here's the formula. I made the beater symbol capital B. I already used center I already used up the letter C for center, so I have to use a different symbol for chaser. I can't just call it C because that was center for my basketball team. So I'm going to make the chaser Ch. Keeper I made capital K and seeker I made S. So if I write the formula for a Quidditch team, I have two beaters, three chasers, one keeper, and one seeker. But remember the ones are understood, so I can just drop those, and I'm left with the formula of the team, is B2Ch3KS. So with that formula I can do all kinds of calculations about Quidditch. Okay, for example, I could do a crazy calculation. Here's a crazy question. How many chasers, and you probably can do this in your head quickly, how many chasers are needed to build 107 Quidditch teams? To do this, you probably need the formula, right? Because the formula was the key. So remember, we had, on a Quidditch team we had two beaters, three chasers, one keeper, and one seeker. Go ahead and do the calculation. Okay, I think that's enough analogies. Let's get on with making calculations of chemical molecules using stoichiometry. So earlier we said that the atom was the smallest individual unit of an element. But the atoms can combine to form molecules. Here I've made a nitrous acid molecule. It's got the formula HNO2. If I wanted to build just one nitrous acid molecule, I would need to have one hydrogen atom, one nitrogen atom and two oxygen atoms. With those pieces, I can build a nitrous acid molecule. And it has a structure that looks like this. I haven't shown the lone pairs, but I've shown the bonds. If I wanted to build three nitrous acid molecules, one thing I could do, is I could write them all down, as I've done here. Or, I could just use the formula to do the calculation. If I put a three out in front of my formula. I see that in order to build three nitrous acid molecules, I would need three hydrogen atoms, three nitrogen atoms, and six oxygen atoms. You see how we can use the stoichiometry that's contained in the formula to do a calculation? Now let's remember my fictional quantity, the tournament. This time I'm not asking how many individual atoms are needed, I'm asking about groups of atoms. The group size is a tournament, which I have defined as eight. To build one tournament of nitrous acid molecules, how many tournaments of hydrogen, nitrogen and oxygen do I need? Well, a tournament of nitrous acid molecules would be eight nitrous acid molecules. Which would mean I would need eight hydrogens because there is only one hydrogen per nitrous acid molecules. But eight hydrogens is one tournament, right? Eight nitrogens would be one tournament. For the oxygens I need two oxygens per molecule, right? And there's eight molecules in a tournament, so I need 16 oxygens in one tournament. Or, another way to think about it is, I need two tournaments of oxygen to build one tournament of nitrous acid molecules. I think we'll stop there. This has been a long enough lecture. So, to review, in this lecture you've learned how stoichiometry relates to basic chemical formulas. Please use this week's exercises to help you gain proficiency at finding and applying the ratios and chemical formulas in calculations. In the next video in this series, I'll work examples with some more complicated chemical formulas. And I will introduce the ubiquitous chemical quantity called, the Mole. So, please check that out soon. We'll be using moles instead of tournaments in the next lecture.