 Okay guys. So let's try this problem. This problem is very similar to the one we just did on the conservation of mass problem. Of course it's the exact same problem without including the mass of water. So we're going to assume we don't know what that is, and we're going to ask what's the mass of Z2 hydroxide, okay? And hopefully, I don't remember what the number we got from last time was, but when we compare them, they'll be the same value, okay? And that will kind of prove to ourselves the law of conservation of mass a different way, okay? So how would we do this? So we've got the mass of zinc oxide, zinc 2 oxide, and we want to know the mass of zinc 2 hydroxide. So we look at the chemical equation here, and we see that there's a 1 to 1 ratio of zinc 2 oxide to zinc 2 hydroxide. Do you guys see that? 1 to 1 ratio. In fact, there's a 1 to 1 to 1 ratio of all of these chemicals, okay? But we're not interested in water, so let's write a conversion factor that describes that, okay? That 1 to 1 ratio. So for every 1 mole of zinc 2 oxide, we get 1 mole of zinc 2 hydroxide. That makes sense, right? Okay, so remember, this chemical equation is just like an algebraic equation, as a matter of fact, and you can set these kind of things equal to each other, okay? So the next thing we need to know is what's the formula weight of zinc oxide, or what's the formula weight of zinc hydroxide? In fact, we've got to figure both of them, okay? Formula weight is calculated the same as molecular weight, okay? Except that, of course, these are both ionic compounds and not covalent compounds. That's why you call it formula weight. So let's figure out the formula weight of zinc oxide. So, of course, there's 1 zinc in 1 oxygen, so it's going to be 1 times the atomic weight of zinc plus 1 times the atomic weight of oxygen. 1 times anything is itself. So if we look up at the periodic table, we see that zinc's atomic weight is 65.39 amu plus oxygen 16.00 amu. That makes sense, right? So let's just add these two together. Okay, when I do that, I get 81.39 amu. Okay, that's the formula weight for zinc oxide. So remember, now we can set up a new expression that's a conversion factor, right? So for every 1 mole of zinc oxide we got, that weighs 81.39 grams. What's next about this expression that we got here? How we did that? Okay, so now let's figure out the formula weight, lastly, of zinc 2 hydroxide. Atomic weight of zinc is 65.39 amu. And remember, you're always going to use that average atomic weight that you find on the periodic table. And then we've got oxygen. Well, how many oxygens are there in this molecule? There's two of them, right? So 2 times that 1 is 2. So it's going to be 2, 16.00 amu. And we're going to add that to two hydrogens, right? Because 2 times 1 is 2. So we've got 2 times 1.00 amu. Okay, so hopefully, I think that's right, right? 1, 2, 2? Yeah. So when we add all those numbers up, we get some other number, right? 65.39, 89.406. And significant digits don't matter yet. They only matter at the hand. Okay, so hopefully you guys got that big number that I got. Make sure your calculation. So now we can set up this new expression. 1 mole of zinc 2 hydroxide equals 99.406. So 99.406 grams of zinc 2 hydroxide. Okay, does that make sense where we got that number from? That expression? Okay, I think that's all we need to do this problem, okay? And if it's not, we'll figure it out during the time we do the problem. Okay, so I'm going to erase out, let's do these formula weights and then go ahead and do the problem in that same space. Of course, we got all that information over here so we don't have to worry about it too much. So, okay, we've got the mass here. We're looking for the mass here. Let's go for it. So we say, well, the mass of zinc oxide is 28.4 grams by the conversion factor of 1 mole of zinc oxide equals 81.39 grams of zinc oxide. Okay, so we'll take 81.39 grams of zinc oxide, put it on the bottom of course because we want the grams of zinc oxide to cancel out and put 1 mole of zinc oxide on the top, like that. So now we're to moles of zinc oxide, right? So we used this expression already so we don't have to worry about that anymore. So what's cool is we've got this expression here that gives us moles of zinc oxide to moles of zinc to hydroxide. So we just use that. So 1 mole of zinc oxide is going to go on the bottom. 1 mole of zinc to hydroxide is going to go on the top, like that. Notice that it cancels out the moles of zinc oxide. We're left with moles of zinc to hydroxide. That's not what we want. We want grams, but it's cool because we used that one. We've got our last conversion factor that converts moles to grams of zinc to hydroxide. Okay, so all we've got to do is multiply that again. Unfortunately, kind of get a little bit more room there. Okay, so moles of zinc to hydroxide are going to be on the bottom because we want that to cancel out. So 1 mole of zinc to hydroxide, that cancels and we put 99.406 grams of zinc to hydroxide on the top and put an equal sign. Why do we put that? Because what are we looking for? The mass of zinc to hydroxide. Is the quantity of grams of zinc to hydroxide? Is that describing the mass? Yes, but we know we can stop there. Okay, so what is the mass of zinc to hydroxide? All we've got to do is multiply it. We'll divide 28.4 times 99.406 and then divide that number by 81.39, 28.4 times 99.406, divided by 81.39, 2, 4, 7, 2, hydroxide. That's the same answer that we got before, is it? Okay, cool. So that should prove to you that you can do it both ways. If you've got both masses here, you can just add those together and get this mass. But if you don't have both masses, of course, you don't know what the mass of water is, so you've got to do it this way that we did it. And you should get the same answer. It was the same answer, right? I don't recall what it was from the first. I guess I shouldn't have done that.