 Alright, one final run-through of stoichiometry before we do our retake, and again I'm going to be using this table that I gave the students that came to review in class. It kind of works your way through the problem, helps you think your way through it without having to think as much. Idea being, you use this to start with, you use it for the first few problems, and then try to win yourself off of it. You try to move away from it so that you can do the process on your own. This is just to help model the thought process when I'm not around there to show you what to do. Now this is kind of tight because of the elevation, the height of my camera here, and the fact that I'm using a wide piece of paper today instead of the normal half sheet, so I can adjust this a little bit. There we go. I'm going to work through this first problem on the sheet using the guide here, and then just like the last one, your job is to do the rest. I'll put the answers up here, but you need to do them before you check them. So first thing we got to do is start by looking for these six key words in our questions. The words we're looking for are grams, liters, molecules, atoms, and formula units. Any time we see one of those six words, we underline it, and any number or substance associated with it. So let's give that a shot. There's our first question, move that down so you can't cheat. Go through the question looking for those words. So I move along, I read along, I scan along, there's moles. I underline it, any number, and substance associated with it. Then I continue scanning along, there's another one of my words, liters, and again I underline it and any number, there's no number this time, and substance associated with it. Now when we're done, we should have underlined two things and only two things, and only one of those two things should have a number. It tells us up in here, you should underline two things, and only two things. One's going to have a number, the other one won't. The one with the number is the given, the one without the number is the unknown. So the one with the number is the given, and write down the unit and the substance. Always write down the unit and the substance, you'll never know what to do in the next step unless you do that. Always write down the unit and the substance. The one without the number is the unknown. Given has the number unknown does not. Now we move on to this little table. We look at our given and unknown and we see how many of them are in moles. That will tell us how many steps to do. If both of them are in moles, it's a one step problem. If one of them are in moles, two step problem. If none of them are in moles, three step problem. Let's take a look. Moles, leaders. One of the two are in moles, and if one of the two are in moles it is a two step problem. Now we got to figure out what those two steps are. To do that, we look at our given and we see what unit it's in. My given is in moles. This little table here says if your given is in moles, then you'll do a mole ratio conversion first. If your given is in moles, then you will do a mole ratio conversion first. So I know step one, mole ratio. Now I got to figure out what to do with my second step. My unknown is in leaders. I go to that table. If your unknown is in leaders, then you will do a mole volume conversion last. So my second and last step, mole volume conversion. Now if this has been a three step problem, we would know our first step and our last step. There's still a step two in the middle. The good news is in a three step problem, the one that's in the middle is always the mole ratio. So you'll know what it is without needing a table. If it's a two step problem like this, the first table tells you the first step and the second table tells you the second step. If it's a one step problem, if you go through all this process, these are both in moles and it's a one step, then do whatever the first table tells you to do. It's going to be a mole ratio, so you really don't need the table. But just do what the first table tells you to do. All right. Now, first step's mole ratio. We need an equivalent. That's where we go to the back side of the sheet here. In a mole ratio conversion, the equivalence is blank moles of the given equals blank moles of the unknown. Blank moles of the given, and the given is that KClO3 equals blank moles of the unknown. O2. Blank moles of the given equal blank moles of the unknown. Fill in the blanks for the coefficients from the balanced equation. So I find KClO3 in the balanced equation. I've got a two in front of it. So I put a two in that blank. Oxygen in the balanced equation. I've got a three in front of it, so I put a three in that blank. That's the equivalence I need to get my conversion started. We always start with the given. In step one, the first thing you write down is the given. 3.5 moles KClO3 times and then the fraction bar for our conversion factor. These are the numbers that go into the conversion factor. Your equivalence is always what goes into the conversion factor. I've got KClO3 as my given. That means I've got to put KClO3 on the bottom. The goal is unit cancellation. I put it down there so it will cancel out. And when my calculation is done, I'll have moles of oxygen left, which is the unit that I was asked to find. Now, I've got to do the math. Multiply by the number on top and then divide by the number on the bottom. So it's 3.5 times 3 divided by 2, which is 5.25. The unit would be what you see here, moles of oxygen. Whatever's left ends up being your unit. And again, it's always writing down unit and substance, unit and substance. Unit, substance. Unit, substance. Unit, substance. Because we need that stuff. I knew I had to put the two moles of potassium chloride, KClO3, on the bottom, because I had it over here. It told me what to do. If I don't write this stuff down, there's nothing to tell me what to do. And then I'm just guessing. Now I move on to step two, which is the mole volume conversion. In a mole volume conversion, the equivalence is always one mole of a gas equals 22.4 liters of the gas. The gas is oxygen, our unknown. What we're left with in trying to convert oxygen. We carry over the answer from this step. That's where we start. And again, when we do this conversion factor, the numbers that go in the conversion factor come from this equivalence. I know what to put on the bottom because I wrote this down. Mole's oxygen. Mole's oxygen. It has to match. That goes on top. Mole's oxygen cancel. We're left with liters of oxygen, which is what we're trying to find. We ignore the one because mathematically it doesn't do anything. The 22.4 is on the top, so we multiply by it. 5.25 times 22.4 is 117.6. That is the answer to the question because its unit matches the unknown. That's how we know we're done. Now you're going to do the rest of these on your own because that's the only way you're going to learn how to do it. And you're going to use that table, that paper with all the little thought process on it to figure out how to get through it. And then when you're done, you're going to come back to this video and you're going to watch it. And you're going to see what the answers are. But don't just copy my work because I'm not giving you an answer to copy on the makeup test. So you've got to be able to do this stuff. The only way you learn how to do stuff is by doing it. It's going to be a struggle. But if you're not struggling, you're not learning. If something's too easy, that means you're not learning anything. You probably already knew how to do it. This is school. We learn. Struggle. Now, if you've done those problems, here are the answers. So, underlined leaders of oxygen and grams of potassium chloride. The one with the number was the given. The one without the number was the unknown. Neither one of these are in moles so I know it's a three-step problem. My given is in liters, so I know the first step is a mole volume conversion. So that's where I get this party started. In a mole volume conversion, the equivalence is always one mole equals 22.4 liters. I take that given and I write it down first. That's where I start. I take that equivalence. I put it as one mole of oxygen over 22.4 liters of oxygen so I can get the liters of oxygen to cancel. I ignore the one, the 22.4 is on the bottom, so I'm dividing by it. 38 divided by 22.4 is 1.7. This is a three-step problem. Step two is always a mole ratio in a three-step problem. So it's blank moles of my given, blank moles of oxygen, equal blank moles of my unknown, KCl. Blank moles KCl. The three and the two came from the balanced equation at the top. Again, I carry over my answer. I set my conversion factor up with the numbers from the equivalence. I started with where I left off, moles of oxygen, so I put the moles of oxygen on the bottom for them to cancel. Moles KCl is the unit for my answer. Multiply by the number on top, so 1.7 times 2. Divide by the number on the bottom, divided by 3. 1.1 moles KCl. Now my unknowns and grams. So I know my last step is a mole mass conversion. For a mole mass conversion, my equivalence is 1 mole equals the formula mass. The formula mass is 74.55 grams. I can carry this over, unit and substance. Because I have moles KCl, I put the moles KCl on the bottom. The 74.55 goes on the top. Ignore the one. It doesn't do anything. Multiply by the number on the top, so 1.1 times 74.55. Question number three. I underline moles and moles. This one had the number on it, so it's my given. This one didn't have the number on it, so it's my unknown. Both are in moles. It's a one step problem. We use that first table, and that first table tells me that when I'm given moles, the first thing I'm going to do is a mole ratio problem. Which would be blank moles of the given equals blank moles of the unknown. Given being KClO3, unknown being O2. And again, those numbers come from the balanced equation. Start with my given, unit and substance. Times in the fraction bar. Because I was given moles KClO3, I put the moles of KClO3 on the bottom. They have to cancel. The number goes on top. Multiply by the number on top. Divide by the number on the bottom. You get 19.2. Unit is what you got left. Moles of oxygen, which is what we were asked to find. So we know that's done correctly. Last one. Four step problem. No, there's no four step problem. It's just a three step problem. Grams. Grams. This one's got the number, so it's the given. That one has no number, so it's the unknown. Nothing's in moles. Three step problem. Our givens in grams, so the first step is a mole mass. One mole equals the formula mass. KCl is 74.55. It's the mass of potassium, which is 39.1. Plus the mass of the chlorine, 35.45. Start with a given, unit and substance. Times in the fraction bar. Because I have grams potassium chloride there, I put the grams potassium chloride in the bottom, so it'll cancel. So that number goes in the bottom. That number goes in the top. All we're left with is moles KCl, which will be the answer. Ignore the one, so it's 425 divided by 74.55. Second step. And a three step problem is always mole ratio. Blank moles of the given equals blank moles of the unknown. And again, those come from that balanced equation at the top of the page. Carry that over. Unit and substance. These numbers go into the conversion factor. Because that says moles KCl, I put moles KCl on the bottom. 5.7 times 3 divided by 2 is 8.55. Moles of 2 is what's left, so that's the unit I write on my answer. Step three. My unknown's in grams, so it's another mole mass conversion. One mole equals the formula mass. 32, not 16, because it's 02. Carry it over. Unit and substance. Because I have moles of oxygen there, I know I got to put moles of oxygen in the bottom, so the one mole goes down there. The other number goes on top. Moles of oxygen cancel. Grams of oxygen for our answer. Ignore the one. 8.55 times 32 is 273.6. I know I'm done because the unit I have on my answer here is my unknown. That's it. If you need more practice, there's a ton on the internet. Search stoichiometry practice problems or search stoichiometry homework or something like that. You'll find a bunch of them. And a lot of them have keys. So just find one that has a key. Try some more. Do it till you get it.