 All right, so this is the stoichiometry retouching session number one that I did on Monday the 27th after Thanksgiving break as we prepare for the retake on the stoichiometry test which will be on Wednesday. You do have to attend one of these unless you are in vocational classes during the second half of the day in order to be eligible to take the retake. So this is what I went over basically. Kind of a way to take some of the thinking anyways out of solving these stoichiometry word problems. I think a lot of the issue is that you see the question there and you don't even know where to start. So I came up with this set of three tables to help guide you through the process and I think if you practice this enough times between now and Wednesday you'll get used to the way the process goes and you'll be able to figure out where to start where to end what steps to do in one of these stoichiometry problems. First thing you get to do is that in a fire given in your unknown in your question and that involves looking for the six key words that I talked about in class. Those six key words are grams, liters, moles, molecules, atoms and formula units. Those are the six key words we look for when reading a stoichiometry question. Remember some of these can be abbreviated moles could be MOL and I know a lot of students are getting that confused with molecules. MOL means moles no matter what any time you ever see it. Liters with an L usually capitalized doesn't look like a one and grams with a G. Those can be abbreviated so if you see the abbreviations those count too. So I've got this example question here. If you have 3.5 grams of iron four sulfide to work with how many liters of oxygen would you need for a complete reaction? What you do is you just go through and look for the key words. When you see them you underline it and any number and substance associated with it and again you scan on. When you see it again another one of those words you underline it and any number or substance associated with it. Bottom line is one of those should have a number one won't. The one with the number is the given. The one that does not have a number the numberless one is the unknown and you use the given and the unknown to determine how many steps you're going to do in the problem and what steps to do. First thing we do is look to see the number of times you underlined moles. Over here we underline grams over here we underline liters. We didn't underline moles at all. We underlined it zero times and that means it's a three-step problem. I would then go on to figure out what steps I have to do. If I had underlined moles twice in other words both of these are moles it's a one-step problem a mole ratio problem. If I underlined moles one time there would be a two-step problem. So again I look to see how many times I underlined moles and again I did not underline moles at all. No moles three-step problem. So now I'm going to figure out what those steps are going to be. For that I move on to these tables. First table tells me what I'm going to start with. What step I'm going to do first and it's all about what the given is in. I look back here the one with the number in it is my given. My given's in grams. So I know the first step in my problem-solving process will be to do a mole mass conversion. The next table tells me where to end the process. What my last step is going to be. The first table is the first step. The second table is the last step and now I'm looking at the unknown. To see what unit that's in I go back up to my question here. My unknown is in liters. Because my unknown is in liters I know I will end with a mole volume conversion. That'll be the last step I did. So the question is what do I do in the middle? When doing a three-step problem you always do the mole ratio conversion to step two. So I know my first step is a mole mass. My second step always the second step in a third three-step problem is mole ratio and my third step in that problem would be to do a mole volume. Those are the three steps I'm going to do to solve that problem. Now we have to remember our equivalences because we need equivalences all the time when we do this stoichiometry. Whenever I'm doing a mole mass one mole of my substance is equal to the formula mass or molar mass as it's sometimes called in grams. When I'm doing a mole volume one mole of my gas equals 22.4 liters of that gas. When I'm doing a mole particle and that is when you're doing the mole molecules mole atoms formula unit one this one here one mole equals 6.02 times 10 to the 23rd. And then finally mole ratios when I'm doing those blank moles of the given equals blank moles of the unknown where you fill in those blanks with the coefficients. So you look at your balanced equation you get the coefficients out of that balanced equation. So again in solving this problem I've already decided that this is a three-step problem and I've decided it's a three-step problem because my given is 3.5 grams of iron for sulfide. Now should I panic if I don't know what iron for sulfide is? Absolutely not. When you look up at the balanced equation there's only going to be one thing that has iron and sulfur in it that is iron for sulfide. My unknown was the one that had no number. Blank liters of oxygen. And if I forget what oxygen is again I just look for it in the balanced equation up there O2. By the way that equation was not balanced. The coefficients are 8, 11, 2, 8. Once that equation is balanced that's a pretty good balancing problem there. Lots to think about. Lots to do in that one. 8, 11, 2, 8. Those are my coefficients. They're not on the paper already. Again I looked at these. Nothing was in moles and when nothing's in a moles it's a three-step problem. And I know automatically my second step my middle step in a three-step problem is mole ratio. I just need to figure out my first to my last. My given was in grams and when your given is in grams you do a mole mass conversion first. My unknown is in liters and when my unknown is in liters I do a mole volume conversion as my last step. So for the mole mass conversion one mole equals the formula mass. I'm always starting with my given. So it's the iron for sulfide. You calculate the formula mass and if you calculated it's 119.99. I'm not going to calculate the formula mass. If you need help calculating formula masses go back and watch my video on formula mass. Start with my given. 3.5 grams FES2 the iron for sulfide times and then the conversion factor. Because I was given grams I have to take the grams from my equivalence and put it on the bottom and I take the one mole and put it on top. I do this for unit cancellation. I need my grams to cancel out. So it's 35 grams times 1 divided by 119.99. Now obviously I can ignore the times one part. Any time I have one in one of these conversion factors I can ignore it. So all I'm going to do here is divide by the 119.99 and the reason why I divide by it is because it's on the bottom. So 3.5 divided by 119.99 is rounded 0.03 and I'm not even bothering with significant figures here because that's not important at all. We're not scientists here we're just trying to learn stoichiometry. I want to get these problems set up I don't care about it really. So 0.03 is where I'm rounding that because that's an easy place to round it. Step two mole ratio and again for mole ratio the equivalence we use is blank moles of the given equals blank moles of the unknown. My given is FES2. My unknown is oxygen O2 given unknown. I find them in the balanced equation to find the coefficient to stick on them. FES2 has an 8. O2 has an 11 and those numbers come in from the balanced equation. This answer carries over from the first step times in the conversion factor because I'm starting with the iron 4 sulfide the FES2. I have to put the iron 4 sulfide the FES2 on the bottom. It's all about unit cancellation. I'll put the 11 moles of oxygen on the top get those moles of FES2 to cancel multiply by the number on the top divide by the number on the bottom. So 0.03 times 11 divided by 8. I'm rounding. 0.04 and again notice I'm writing down the units and the formulas with everything. I'm not skimping. I'm not cutting. I'm writing down everything and you should too. Step three the final step we already decided is a mole volume conversion because we need liters. Whenever doing those one mole of your gas equals 22.4 liters of that gas. I can carry over the answer from the previous step and moles and moles. They're all the same. MOL. We don't abbreviate molecules. If it were molecules I'd write out the word molecules. The moles of oxygen so I put moles of oxygen on the bottom. The 22.4 liters on top moles cancel. I ignore the one 22.4 is on the top so I multiply it. I get 0.896 and again I'm just going to round to 0.9 liters of oxygen. Again it's a thought process. You've got to figure out where to start and where to end and maybe those tables will help you. I don't know it's a shot. Give you something to look at while you're practicing and then once your practice is enough you'll get it and when it clicks it clicks. It's one of those things where once you know how to do it you can do it. Now I'm not going to go through the other two step by step. I want you to practice those so if you have not done those yet then stop the video. Do yourself a favor and do some practice. Watching me do this doesn't mean you can do this. You got to figure it out. You got to struggle with it. You know if you're not struggling you're not learning. If you're doing something in class and it's easy that means you probably already know how to do it so you're not learning. If you're struggling you're learning. So work at it, struggle at it and try to get it. Once you're done come back and watch the rest of this video. Now I'm not going to work these out. I'm just going to run through them real quick. There's my second one. My second one again I didn't have any moles in it anywhere. My given was in liters and my unknown is in grams. So I know this is a three step problem and again the mole ratio is always in the middle. My given was liters so I know my mole volume conversion goes first. My unknown is in grams so I know I end with a mole mass. Now you can see all the math involved. With a mole volume it's 1 mole equals 22.4. With a mole ratio it's blank moles of the given. The given was oxygen equals blank moles of the unknown. So too and again if you don't know what sulfur dioxide is you look at the equation at the top of the page. It's the only thing with sulfur and oxygen in it. Those numbers come from that balanced equation and then finally mole mass 1 mole equals the formula mass. The formula mass of sulfur dioxide is 64.07 and again if you don't know how to calculate a formula mass go watch my video on formula mass. There is the answer to the second one. This one's a two step problem. I have moles for my given. I have grams from my unknown and again when we look back at the table if one of them is in moles it's a two step problem. And now we can use those tables the way that they're supposed to be used to figure out where to start and end. Because my given was in moles because my given was in moles I know I start with the mole ratio problem. The mole ratio step. My unknown is in grams and because my unknowns and grams I end with the mole mass conversion. So again those little tables will tell you where to begin and end your two step problem. If it is a one step problem it is mole ratio. Bottom line. Moles first if you're given moles you do the mole ratio. There it is. Blank moles of the given. The given is Fe2O3 equal blank moles of the unknown. The unknown was FeS2. Work that all out. You'll be in moles of FeS2. Then we do a mole mass and again one mole equals the formula mass and once again if you don't know how to do a formula mass go watch my video on it. Always setting up for unit cancellation in these. So this is the head moles here. I put moles in the bottom. I didn't cross them out that time but again it's all about the unit cancellation. Moles, moles, FeS2, FeS2, all that stuff cancels out. We got to make sure we do our cancellation and our setups. I have one more set for you tomorrow. I am not going to post a video on tomorrow's sets. I do need to see some more people come in here because you do have to attend a session to do a retake.