 Okay, so let's do this problem on the law of conservation of mass. So it says, aqueous solutions of mercury-2 nitrate and potassium iodide will react to form a precipitate of mercury-2 iodide and aqueous potassium nitrate according to the reaction that's written on the board. So if you add 3.25 grams of mercury-2 nitrate to 3.32 grams of potassium iodide, what is the total mass of the reactants? Okay, so potassium iodide is here, so 3.32 grams. So remember which, what are the reactants in this chemical equation, guys? The ones on the left side. The ones on the left side, okay, so all we have to do is add these two together, right, so those are the reactants. And both of those numbers are given to us. So let's just put them in our calculator, 3.25 plus 3.32 is 6.57 grams. So that's the mass total of the reactants. Is everybody okay with that? It says then, what would be the total mass of the products after the reaction? What would that be? The same. The same, right? What's that number? 6.57 grams, right? 6.57 grams. So in other words, the total here, the total here mass of the products is 6.57 grams as well. How do you know that? Because of the law of conservation of mass, right? So the law of conservation of mass says that matter can't be created or destroyed. The additive sum of the reactants should equal the additive sum of the products, masses. Is everybody okay with that? And then the last problem says, if there was 4.55 grams of mercury-2 iodide formed, what would the mass of potassium iodide be? So it says 4.55 grams of this stuff. Oh, potassium nitrate, I'm sorry. What would the mass of potassium nitrate be? So how would we go about figuring that out? Subtraction. Subtraction, yeah. So we would take the mass total, so in other words, let's just write this out. So the mass total here is going to be the mass of mercury-2 iodide plus the mass of potassium nitrate. And we got the mass total. Sorry, we're looking for the mass of potassium nitrate, so let's just rearrange this equation. So that's going to equal mass totaled minus the mass of mercury-2 iodide. And you don't have to do all of this, of course, but it's nice to look at the actual formulas that you use. So we got 6.57 grams here. And we're going to subtract, what would we say, 4.55 grams? 2.02 grams. And is that the right number of significant figures? Now why is that the right number? There's two after the decimal points. Both of these have two after the decimal points, so this has two after the decimal points. So that's the mass of potassium nitrate. Is everybody okay with that? And again, like I said, I'll put some on the review sheet for you guys. Questions?