 Okay. So, let's start here with a theoretical yield problem. Okay? So, it says here, the theoretical yield of iron was 30 grams. Okay, so 30 grams of iron. Let's make up a question. How many grams of aluminum did we start with? Okay? Is everybody okay with that? Let's try that problem. So, what did it say? The theoretical yield of iron is 30 grams. So, the mass of iron, 30 points, it actually says 30.0 grams. Okay? So, do iron and aluminum weigh the same amount? So, we got to convert things from mass units to what units? In order to mole units, right? Why mole units? Because mole is an amount, right? It's like counting things. Okay? So, instead of weighing these two things, which would be the practical way of doing it, because in the lab it's easier to weigh things than atoms, especially than to count down, of course. We have to convert from weight to counting back to weight. Okay? So, that's what we're essentially doing. So, how do we convert from mass to moles? You guys remember? So, what we're trying to figure out is the mass of the aluminum, right? And we've got the mass of iron. You guys can help me any time. It's all being recorded. So, molar mass, very good. So, grams on the bottom, right? Grams of iron. One mole on the top, very good. And that's going to be one mole of iron, right? It's good for you guys to put the iron there. Just kind of get these conversion problems because we're going to go to aluminum in a second, you know? So, we've got to make sure that we're talking about iron or aluminum. So, now we've cancelled out grams of iron. But remember, we're looking for mass of aluminum. So, have we gotten that yet? So, what do we got to do now? Mole to mole. So, figure out the moles of aluminum, right? So, what's the mole to mole ratio here? Or, yeah, or a 2 to 2. Let's just keep it, just keep it 2 to 2. Because why? There's a coefficient here is 2, coefficient here is 2, okay? So, 2 mole Fe, 2 mole Al. And you guys are right, it's a 1 to 1 ratio. So, we figured out the number of grams of al yet? No. What do we do? Mole or mass again? What is it? How do we find it? Look at the periodic table, right? 2698. So, the mass of aluminum yet? You guys think you can do that on here? Just remember, this is what chemistry is all about. Once you learn the periodic table and once you learn cancelling units, you should be able to do anything in chemistry. Okay, so let's do it together. 30 times 2 times 26.98. Okay, that number divided by 55.35. That number divided by 2. And what do you guys get? 14.5. 14.5. Why only that many sig figs? Because you started with 30.0. Yep, because you started with 3 sig figs. Congratulations. So, if the theoretical yield, if the theoretical yield of that is 30 grams, then you must have started with this many grams of aluminum. That's what it said. Does that make sense to everybody? Are there any questions before we kill this video? We'll do a percent yield. Okay. Question? Yeah. Why did we use 2 moles of al? Why did we use 2 moles of al? Because it said the chemical equation is balanced to 2 to 2. If you wanted to, you could say a 1 to 1 is the same, right? Because 2 divided by 2, of course, equals 1, right? But I just like to, especially with introductory or general chemistry students, just keep it what's presented on the chemical equation and it really, you know, makes your job a little easier so you don't have to always take ratios and stuff. Does that answer your question? Yeah, you take this from the problem. Yeah, you take it from the actual chemical equation if that's what you were thinking. Okay. Any other questions? Okay, cool. Good job guys.