Music Lesson 11 is sticky on the tree, when there's three of the toppings. So what is the purpose of stoichiometry? We're going to discuss that first. And then we're going to go into the multiple ratios, and then the Ha... How do you put it? And then the Haver Bosch example problem. So, great, great picture you got today. The calculation of quantities in chemical reaction is a subject of chemistry called stoichiometry. Calculations using balanced equations are called stoichiometric calculations. For chemists stoichiometry is a form of bookkeeping. For example, countants can track income expenditures and profits for small business by tallying each in dollars and cents. Chemists can track reactants and products in a reaction by stoichiometry. It allows chemists to tally the amount of reactants and products using ratio of moles or representative particles. The other scenario is sometimes we don't need as many moles as they are given to us in the original formula. So sometimes we want to do things on micro-scales, sometimes we want to do things on macro-scales. So we're going to use these formulas to actually help us to do that. So stoichiometry is also commonly found in all types of video games. Some of the major RPGs out there like Skyrim or Fallout 4 or World of Warcraft or actually any game that requires you to gather items to make something is all using stoichiometry. Stoichiometry we're just figuring out how many items you need to craft the tools or the equipment that you need to progress in your game. And cooking. Cooking is another example of stoichiometry. Ingredients which we can call reactants make dishes which we can also call products. So looking to the left I have this awesome recipe for creamy poblano mac and cheese. Totally trying that out. It's really really good. It's like a nice hot spicy mac and cheese and if you add other things to it it's even better. But let's just assume that this recipe cooks for five people. And we notice how many cups of shredded cheddar cheese you need if you're cooking for ten. So first off if you're going from five people to ten people that means you have to double up all your recipes. In our original recipe for five you need one cup of cheese. For ten I'm going to assume you need two cups of cheese. Otherwise every person would get half the amount of cheese that they normally would get in their mac and cheese which is just simply pasta with a little bit of cheese which is more. Now the rest of the ingredients if you're doubling up just the cheese do you leave every other ingredient the same? No you have to double those up too. Again stoichiometry is taking the entire chemical reaction and changing it for the needs that you have. The Haber Bosch industrial process synthesizes ammonia by the reaction of hydrogen and nitrogen. Ammonia is commonly used as a fertilizer and can be also found in windex. So this formula when we balance it if you remember from the earlier videos comes out to be a one to three to two ratio of these reactants to products. Remember that we have to abide by the conservation of matter and mass law and that we really do need a balanced chemical equation in order to do our stoichiometric calculations. So we're going to use this as our examples. So when we have a balanced chemical equation we end up with 28 grams per mole of nitrogen gas, 6 grams per mole of hydrogen gas. And then we end up with 34 grams per mole of the ammonia. Now remember conservation of mass which means that the reactants 28 plus 6 has to equal the products which is 34. So we answer these questions. We need to find something called a mole ratio of the balanced chemical equations. A mole ratio is the permanent ratio derived from the coefficients of a balanced chemical equation in terms of moles. So again we have one mole of nitrogen gas, three moles of hydrogen gas, two moles of ammonia gas when we balance this equation out. Which means we have a mole ratio of one mole to three moles to two moles. Mole ratios are used to convert between the moles of the reactants and the moles of products between the moles of just the reactants or between just the moles of the products. So in our steps to stoichiometric equations the first thing you have to do is write down a balanced equation and determine the mole ratios from the equation. Remember this is a permanent ratio so any alterations will affect the other values in the same proportions. It's like taking a cooking recipe and forgetting that you have to add something or forgetting the correct amount of cheese for your mac and cheese. The second step is to identify the known and the unknown in the problem assigned. Not all the substances need to be used or not all the substances are described. So sometimes they will talk about things in your problems but they're not important. We're going to ignore them. The last step is we're going to set up a simple proportion and solve for the unknown. So in our Hava process we're noticing that we're again taking nitrogen plus three moles of hydrogen and it's going to give us two moles of our product. So how many moles of ammonia which is at H3 are created when 0.4 moles of nitrogen is used. The first thing we do is we analyze our bounds for them. We have a 1 to 3 to 2 ratio. The second step is to analyze and say which other things that we're going to be talking about. We're looking at nitrogen because it's in the question and we're also looking at ammonia because it's in the question. So when we make proportions we're going to be ignoring all the information about hydrogen. We're going to pretend it's not even there. If you can see it already we already have our proportions given to us. So the last step is to simply cross-multipline solve for x. So one mole of nitrogen over 0.4 moles of nitrogen is equal to two moles of ammonia over x. When you cross-multipline solve you should get 0.8 moles of nitrogen being produced. Does this make sense? It should make sense because you have less than one mole of nitrogen being made which means you have less than what you started with. So you should end up with less than when you end with. Now the other thing is that hydrogen gas is changing. However the question does not ask you for any information on hydrogen which is why we're just ignoring it. It's not that it's not changing. It's just being ignored. So in this next example we're going to try it out with how many moles of hydrogen gas are needed to produce 1.6 moles of ammonia. Again first determine your mole ratio from your balanced equation. We have one mole of nitrogen, three moles of hydrogen creating two moles of ammonia. Then you add the information that is given to you. So we're talking about moles of hydrogen but we don't know how many so that's going to be x. And then we have 1.6 moles of ammonia. So we put that right underneath the original amount of ammonia. Notice that we're ignoring in this case the nitrogen gas. We now set up that proportion that you can already see. So we have three moles of hydrogen over x equals two moles of ammonia over 1.6 moles of ammonia. So now cross-multiply and we end up with 2.4 moles of hydrogen. Does this make sense? It should because if you're noticing you got it from two moles of ammonia to 1.6 which is a decrease. Instead of recipes for five people you need to get a recipe that will only cook for four people. So if you originally had three moles of hydrogen you need to have less moles now. In this problem we're again looking for our reactants. We now have the same processes before the other process. But we are going to be looking for how many moles of nitrogen we are using when we only use 2.3 moles of hydrogen. So again first step is to find our ratios which is a 1 to 3 to 2. Second step is to look the problem. We're looking for x which is the moles of nitrogen. We are given 2.3 moles of hydrogen. So this is a reaction between reactants and reactants. But we don't care how much ammonia we're producing right now. So you're looking at your proportion which is step three which is one mole of nitrogen over x is equal to 3. x is equal to 3 moles of hydrogen over 2.3. Now just looking at our givens if you have 3 moles of hydrogen it only 2.3 moles from the problem. This means that our x should be less than 1 mole. So if you cross both points out you get an x of 0.78 moles of nitrogen gas.