 So let's do another conservation of mass problem. So this one says a sample of mercury with a mass of 114.0 grams was combined with 12.8 grams of oxygen gas. And the resulting reaction gave 123.1 grams of mercury to oxen. How much oxygen was left over after the reaction was complete? So I know you guys haven't balanced any reactions yet. So let me just write this reaction on the board. So it says that the sample of mercury had a mass of 114.0 grams was combined with 12.8 grams of oxygen gas. The resulting reaction gave 123.1 grams of mercury to oxide. What did I say? 121.3 grams of mercury to oxide. How much oxygen was left over after the reaction was complete? So we know that all of this was reactive, because they're telling us some oxygen was left over. So since they're telling us that, eventually you'll learn how to figure out what's called the limiting reagent. At this point, you don't understand that yet. So this is for people watching the video that are like, they just need to figure out the limiting reagent first. But anyways, they're telling us that this is the limiting reagent. Or sorry, this is the limiting reagent. This one is an exonus, because they're telling us some is left over. Is everybody OK with that? Anyways, so what they're saying is all of this was reacted. So if we got this much product, all we really have to do to figure out how much oxygen is left over is just take the mass of the product and take the mass of mercury. The mass of the product is the mass of mercury plus the mass of oxygen that was reacted. So we want to know how much oxygen is left over. We'll say, well, what's the mass of oxygen? The mass of product minus the mass of mercury is going to be the mass of oxygen. So 123.1 minus 114, we get 9.1 grams. So 9.1 grams is how much oxygen was reacted. Does that make sense? Just extract that from the total. And then, yeah, you're going to take that from the 12.8 and figure out how much is left over. Because that's what you're wondering, right? Oxygen left over or unreacted, I can't remember exactly how they said it, was going to be the mass of O2 total is going to be the mass of O2 reacted plus the mass of O2 unreacted. So we know these two. So mass of O2 unreacted, total minus mass reacted. And so that's 12.8 grams minus 9.1 grams. We're in significant figures, of course, 3.7 grams of O2 of this unreacted. Makes sense? So again, it's just all you're doing is this side, the mass of this side has to equal the mass of this side. And what we figured out is if you add these two numbers up, they don't equal this number. Does that make sense? I mean, there's an easier way to do this. Do you add those two numbers up and subtract that number? Yeah, you could do it that way. That's the easiest way to do it. Do you want to do one? You want to do it, let's do it again. And we'll do it a shortcut. We'll do it in a shorter way. But it doesn't go over step by step by step exactly how you do it. But we'll do the same.