Chemical Reactions (7 of 11) Stoichiometry: Grams to Moles - OK, this is one more in our series that everybody's favorite chemistry topic-- stoichiometry. And this one, we're going to do grams to moles. So we're going to be given the grams of one of the substances in our chemical equation, and we're going to figure out how many moles we need to produce that many grams or how many moles of the other product we're going to produce. So we're going to be given 1 gram, and we got to figure out the rest in moles. All right, so let's go ahead and look our chemical equation. You can see here, we have a single replacement of aluminum and copper chloride. We're going to be given 8.25 grams of copper chloride, and we want to know how many moles of aluminum we need to react to that many grams, and how many moles of copper, and how many moles of AlCl3-- aluminum chloride-- we're going to produce. OK, please remember that the chemical equation is a molar relationship, not a mass or a gram relationship. You can see, it basically says here that 2 moles of aluminum plus 3 moles of copper chloride will yield 3 moles of copper and 2 moles of aluminum chloride. You can think of it as atoms, or particles, or atoms, or molecules, but I like to think of it as moles because that's usually the way we work with it in the lab. So you remember that we said there are three steps to every good stoichiometry problem. And in case, we're going to use two of them, but let's just run through all three. The first step is we're going to convert from grams to moles using the molar mass. So we've been given grams. The chemical formula is a molar relationship, so we have to convert this from grams to moles So step two-- we're going to use the molar ratio between the copper chloride and the other three to determine the moles of the substance that we're using that need or produce. And if we wanted to-- but we're not going to in this video-- then we could convert the moles into grams using the molar mass. OK, so as I said, in this video, we're just going to do steps one and step two. In previous videos, we've gone through the other steps. So you can see those if you'd like. But let's go ahead and get started. And we said step one is to convert from grams to moles. So we're going to convert 8.25 grams into its corresponding moles. Here, we wrote down, as you should, the number, the unit, and then the chemical formula. Of course, we're going to use the molar mass. So the molar mass of copper chloride is 134.45 grams. That is equivalent 1 mole. So here we have our molar mass, which allows us to cancel. We put the grams on the top over here, put the grams on the bottom over here. And that will allow us to cancel. And now we're in moles of CuCl2. OK, so that's the number of moles. And I like to write it up here, so I can see that these two go together. This is the mass. This is the corresponding number of moles. And now, we're going to use this number of moles to do a molar ratio and convert from moles of CuCl2 first to moles of aluminum, then to copper, and then 2AlCl3 aluminum chloride. And we're going to do that on the next slide. But we're going to use that value-- 0.0614-- three times, all right? So let's go ahead and do that. OK, so here is our mass of a copper and our corresponding moles that we were given for copper chloride. And we're going to use that-- see, I wrote down the number, the unit, and the chemical formula. And now, we're just going to use the molar ratio. This would be step two in our three-step process. We want to get out of the moles of copper chloride, so that goes in the bottom. We want to get into the moles of aluminum. We'll do that one first with the 2 to 3 ratio. So you can see, we put the 2 in front of the aluminum from the coefficient up here. The 3 goes in from the copper chloride, and that allows us to cancel moles of copper chloride, and that tells us now we're in 2 moles of aluminum. And that means it's 0.0409 moles of aluminum. So that tells us how much aluminum. So in order to react 8.25 grams, or this many moles, we would need 0.0409 moles of aluminum. Now we're going to do the same thing three-- not three more times but two more times, this being step two, using our molar ratio, we start always with the moles-- not always, you don't have to. But I think it's good just to start with the moles-- the substance you've been given. We could use this one, of course, but let's just be consistent. We know the moles of copper chloride goes on the bottom. The moles of copper goes on the top. And you can see, this is a 3 to 3 ratio-- basically 1 to 1. So the number of moles of copper chloride and the number of moles of copper produced are the same. They won't be the same mass because they have different molar masses. But we're going to put 0.0614 moles of copper. OK, and we just have one more. We're going to start again with the 0.0614 moles of copper chloride. Write down our railroad tracks-- copper chloride in the bottom, aluminum chloride in the top, molar ratio 3 to 2, 2 to 3. And we can cancel the moles, and we see that that is going to be 0.0409. And you'll notice that this value and this value are the same because this is a 2, and this is coefficient of the 2. So it's basically 2 to 2, or if we reduce that, a 1 to 1 ratio. OK, so the first thing we did on the first slide was convert 8.25 grams into its moles. And then we used the same number of moles of the copper chloride and the molar ratio between the copper chloride and the other three-- the aluminum copper and the aluminum chloride-- to convert into the number of moles, that being step two. We're not going to convert from moles to grams in this video. But we have done that in previous videos, which you can watch. All right, so thank you very much. I hope that was helpful. if you also thought that was helpful, then maybe give me a thumbs-up down below in the comments section. And thanks for watching. And we will see you next time.