Hi again, and welcome back. In this video I'll continue relating stoichiometry to chemical formulas. And some more complicated types of molucules will be considered. In addition, I hope to demystify the concept called, The Mole. Sometimes when I meet new people and they find out I teach chemistry, they say, wow, I really loved chemistry when I took that class in high school. But I was always confused about the concept of the Mole. So I think that after today's video, the idea of what a Mole is will be more clear to you. A frequently-used quantity in chemistry is 6.022 times 10 to the 23rd of something, and that's one mole. It was defined in chemistry as the number of carbon-12 atoms in exactly 12.000 grams of carbon. In other words, carbon was used as the standard. The atomic mass units of carbon relate to the grams of carbon in a certain way and in twelve grams of carbon there are 6.022 times 10 to the 23rd, carbon atoms, but in fact, you can really just think of the mole, as a number. There are 6.022 times 10 to the 23rd of anything in a mole. And this is an important number that we use in stoichiometry. So a mole, really, don't worry too much about how it relates carbon atoms. Think of the mole as a quantity. Just like we have other. Common quantities that we use. We might say, we have a pair of shoes. And a pair would be two. Course there are 12 in a dozen, and we might say there's 12 cookies in a dozen. So in a mole, there's 6.02 times 10 to the 23rd items. In a mole. So there's 6.02 times 10 to the 23rd atoms per mole. There are 6.02 times 10 to the 23rd molecules per mole. If we were talking about trees, we could say there are 6.02 times 10 to the 23rd trees per mole So it doesn't really matter what we're talking about. I think that's one of the things that confuses people about the mole, is that we can use the mole to describe the mole number of atoms in a mole or the number of molecules in a mole, or the number of trees in a mole. In fact, I would like to have a mole of pennies. That's a lot of pennies. How much money would that be? Well, in US currency. Let's write it out. That would be the number right there. 6.02 times 10 to the 23rd, but of course, pennies are only one one hundreth of a dollar so I have to put the decimal point in there. That's such a large amount of money that it's unfathomable. There's a trillion right there, that's the number trillion. So a mole is much, much larger than a trillion, which is a number that's so large that I can't really even comprehend it. So mole is a huge number. A mole of pennies would, would bury the earth, probably. In contrast, if we think about the copper atoms that are used to make those pennies. This container has a mole of copper atoms in it. Now, certainly I can't fit 6.02 times ten to the 23rd pennies in this container. But that's how many copper atoms are in the container. Can you see the little balls of copper there? There are 6.02 times 10 to the 23rd copper atoms in this container. That's how tiny atoms are. They're very, very tiny. They're infinitesmally small. They're so little that it's hard to comprehend. We can actually see them now, using an instrument called an atomic force microscope. But for a long time people just had to take the leap of faith that atoms really existed. Let's look at a different type of atom. Here's a mole of sulfur atoms. Now the mass of a mole of sulfur atoms is different than the mass of a mole of copper atoms. That's something we'll talk about more, that's another stoic chronometric concept that we'll be dealing with in this course. So this is atoms, but you can also have moles of molecules. Remember I said that? This container has one mole of water molecules. You see, that's very tiny amount of water. This wouldn't even really quinch my thirst if I drank it, actually. We can talk about more complicated types of molecules. For example, in this container there's this orange powder. I'm from Denver, if you're in the United States you know the Broncos are orange. That's kind of Bronco orange, and that's one mole of potassium dichromate. It seems like it's a lot more than one mole of water, well for one thing there's more atoms. Potassium dichromate has eleven atoms. In each unit, where as water only has three two hydrogens and one oxygen, but not only that the potassium dichromate atoms are heavier. We'll talk about the concept of density in another lecture. What about some molecules that are familiar to you in your everyday life? This is a mole of sucrose. Which is table sugar. It's made of carbon, hydrogen, and oxygen. A mol of sucrose weighs 342.1 grams. You can see there, it's a white powder. But, there's quite a lot of sugar in a mol, isn't there? Because there are so many atoms. It's a heavy molecule. Table salt, on the other hand. If we compare these two containers, you'll see there's quite a bit less volume of the table salt in a mole. But that's really only because the table salt is simpler. It's only got two atoms, okay? It has a different density than sugar. But there are 6.02 times 10 to the 23rd Salt molecules in this container, and there are 6.02 times 10 to the 23rd sugar molecules in this container, same number of molecules in both containers. One mole. Alright. Now, let's move on. Okay, Let's move on to relating the moles to chemicals. Last video we used the example of Nitric Acid molecules, remember we calculated how many Hydrogen's, Nitrogen's, and Oxygen's we would need to build. One nitrous acid molecule or two nitrous acid molecules, and we even did a tournament of nitrous acid molecules. Remember that? I made up that a tournament was eight. But, in this case, we can say, how many hydrogens, oxygens and nitrogens will we need to make up a mole of nitrous acid molecules? It would really to write down that many nitric acid molecules. So, to build one mole of nitric acid molecules we would need 6.02 times ten to the 23rd hydrogen atoms which we can just say is one 1 mole hydrogen atom. We'd need 6.02 times ten to the 23rd nitrogen atoms which we could simply say is one mole of nitrogen atoms and we'd need twice as many moles of oxygen because remember the formula there's a little two in the stoichiometry. So we would need 12.044 times ten to the 23rd oxygen atoms or two moles of oxygen atoms. Now the thing about the quantity of a mole is that you can have fractions of a mole. Just as you can have a half a dozen cookies you could have some fraction of a mole. What is we wanted to have 0.75 moles of nitro acid molecules. The formula is the key again. We can calculate how much hydrogen, nitrogen, and oxygen we would need. We're going to write ratios again using the formula. So if I want to figure out how much hydrogen I would need, well I'll say okay, I'm starting with 0.75 moles of nitrous acid. And I have one mole of hydrogen per one mole of nitrous acid molecules. That's the ratio, that's the stoichiometry. And I can multiply those two things together and see that in order to build 0.75 moles of nitrous acid molecules, I would need 0.75 moles of hydrogen. That's pretty simple. We can do a similar calculation with Nitrogen as you see. Again, my moles of molecules are cancelling out and I'm left with moles of atoms. Becomes a little more complicated for the case of the Oxygen because now I have two moles of Oxygen, remember that comes from the stoichiometry per one mole of Nitric Acid molecules. So now, in 0.75 moles of nitrous acid molecules, I have 1.5 moles of oxygen atoms. Nitrous acid is a pretty simple molecule. Let's consider a more complicated molecule. Let's consider calcium nitrate. Calcium nitrate has this formula that has some parentheses are in it. How do the parentheses effect the stoichiometry? In other words, if I wanted to build five mols of calcium nitrate, what would the ratio be of calcium to molecules calcium nitrate? Of nitrogen atoms to mo, moleclules of calcium nitrate? And of oxygen atoms what would those numbers be? If we look at the formula, we can see, that there's one calcium, because the calcium's not in the parentheses, so we assume that there's a single one there. There's one calcium per calcium nitrate molecule. And then we need to take this two that's outside the parenthesis and multiply it times the numbers that are inside the parenthesis. So, there an understood one next to the nitrogen and the three next to the oxygen. So, every calcium nitrate molecule contains two nitrogen atoms And six oxygen atoms. And I'm multiplying the number of oxygen atoms in the nitrate polyatomic ion times the fact that there have to be two of them in every calcium nitrate molecule. Now let's do the calculation. How many calcium, nitrogen, and oxygen atoms are needed? To build five moles of calcium nitrate. Well, with the stoichiometry, I have these ratios that I can use sort of as convergent factors. I' can figure out then that to build five moles of calcium nitrate molecules, which is a huge number of calcium nitrate molecules, remember, I would need five moles of calcium atoms >> There are two moles of nitrogen for every one mole of calcium nitrate molecules so if I do that math you see I need 10 moles of nitrogen atoms and finally there are 6 moles of oxygen atoms for every mole of calcium nitrate molecules Therefore, when I do that multiplication, 5 times 6. I see that to build 5 mol Ca(NO3)2, we would need 30 mol O. We can do a similar calculation For fractions of mols of calcium nitrate, it doesn't really matter what this number is here. Right? We're always starting with that number. How many mols are we trying to build, right? And then we're using the ratio we're getting from the formula. These ratios come from the formula, right? And then we're doing some multiplication. To calculate how many moles of each type of atom we need to build a certain number of moles of calcium nitrate. So if I had 0.75 moles of calcium nitrate, that sample, that sample of 0.75 moles of calcium nitrate would contain 4.5 moles of oxygen atoms. And I did the math the same way I've been doing it previously. So now let's review. Stoichiometry is simply a matter of using ratios to do calculations and chemistry. Atoms combined to make molecules in specific whole number ratios. And that's one of the things that Dalton figured out in his atomic theory. We can do simple calculations from molecules and we can use this quantity, the mole, to have an amount of molecules that is reasonable for us to weigh out. It would be difficult for us to weigh out one or two molecules because, they're so tiny. But, we could weigh out one mole of molecules on a scale. And that's an amount that chemists can work with. That's practical. That's why we use the mole. So I hope that this week you'll be able to use the exercises to practice this concept of the mole and some stoichiometric calculations. Thank you, and I'll see you in the next lecture.