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Molecular and empirical formulas

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Video transcript

- We now have a respectable understanding of the periodic table itself and the atoms in them, and now we're ready to deal with molecules themselves and to deal with molecules we have to have some way of representing them and you represent them with formulas. There's two major, actually three major ways to represent a molecule. One is the molecular formula. The other is the empirical formula and I'll do it in a different color to differentiate it. Empirical formula. And the difference is, well let's just talk about what the word empirical means. I remember when I first took chemistry the teacher kept using the word "empirical". I said, "What does empirical really mean?" I clearly did not have a very deep vocabulary. I forgot what age it was, but it means "Achieved through observation", or "Experiment", or "Based on experience". So, if someone says that they empirically figured out x, y, or z, it means that they figured it out through an experiment or they observed it. The molecular formula is essentially the actual number of atoms in that molecule. Let me show you what I'm talking about. So the empirical formula tells you what people have observed, maybe before they even knew that there was such a thing as atoms. What they would've observed is the ratio of the atoms to one another in a molecule without knowing in the exact molecule how many of that atom there are. Let me show you what I'm talking about. So if I were to give you a Benzene... The molecular formula of Benzene, you have six carbon atoms and you have six hydrogen atoms. Now, if you were some chemist in the 1800's and you didn't know about the actual atoms but you had a big bag of Benzene and you were to measure the ratio of the carbon to the hydrogen that you have in that bag, you would find out that for every one carbon you have one hydrogen. So your empirical formula is the ratio of the two. You don't know that each atom actually has six of these. But you know that for every carbon, there's a hydrogen. For every hydrogen, there's a carbon. The way to go back, you can go from the molecular formula to the empirical formula very easily. You just find the greatest common divisor of the number of atoms in the molecule. So, the greatest common divisor of six and six is obviously six, so you divide both of these by six and you get the empirical formula. It's not easy, or you pretty much can't go back from the empirical formula to the molecular formula. You've lost information. I don't know whether this was C six, was it C two H two, you just don't know. I mentioned right at the beginning of the video that there's a third way to represent molecules. That's the structural formula, and we'll do that off and on and we've already done it a little bit. Let me show you. The Structural formula for Benzene would actually say how the molecular formula atoms are configured. So Benzene in particular is very interesting It looks like this, it's often drawn like this. You'll see this a lot when you take organic chemistry. It looks like a little hexagon where the vertices of the hexagon are carbon atoms. Let me draw the carbon atoms in yellow. This is carbon, carbon, carbon, carbon, carbon, carbon. They have double bonds every other carbon. Double bonds. And then they have single bonds to hydrogen. Let me just do the hydrogen in another color. Let me do it in magenta. Hydrogen. Obviously the structural formula gives you the most information. Then you can start to think about how this will interact with other things while the molecular formula just tells you what's in the molecule. The empirical formula really gives you the least information. It just tells you the ratio of the different atoms in the molecule. Structural formula. Let's do a couple more. Ok, what if we're dealing with water. I think you know the molecular formula for water. H two O. Now what would be the empirical formula for this? Well we want to know the ratio, so for every oxygen there's two hydrogen, or you could say that for every hydrogen there's a half of oxygen. So you really can't reduce this. If I wrote this as H two O one, what's the greatest common divisor of two and one? It's one, so you just have to divide them by one. In this case, the empirical and the molecular formula are the exact same thing. It's H two O. What about sulfur? Sulfur is an interesting molecule, ^because obviously it's just one atom. ^I'm spelling it wrong. ^It's not a "ph", it's an "f". ^Clearly I shouldn't be making spelling videos. ^So, sulfur. ^So the molecular formula, S eight. ^So it forms this neat kind of octagon looking ^chain of sulfurs, and if I were to draw that ^you would see that, and you can look it up ^on Wikipedia if you like, but ^its empirical formula, if you had just a bag of sulfur ^you don't know that each atom has eight sulfurs. ^You just have a big bag of sulfur. ^So the empirical formula there's only ^one atom in this molecule, you divide by eight ^and you get S. ^So you just know that ^all you've got there is sulfur. ^So let's just do one more. Glucose. We'll have to go a new color. Glucose, the molecular formula is C six, H twelve, O six. So for every carbon, there are how many... For every six carbon there's twelve hydrogen and one oxygen. So, if you kind of reduce this formula to its empirical form, what do you get? We can divide all of these numbers by six. So we get one carbon, two hydrogen, and one oxygen. So this just tells you the ratio that they exist in a big bag of this molecule. This tells you the exact number of atoms in that molecule, fair enough. So now we know a little bit of the difference between molecular formula, empirical formula, and structural formula. Now let's see if we can use what we know about the formulas and the periodic table to think a little bit about the composition, the mass composition of some of these molecules. So, the first thing to even think about is how do you figure out the molecular mass. I've got a little periodic table down there. So, molecular mass. So the first question is, how do you figure out... I mean the molecular mass is going to be the sum of all of the atoms in that molecule, right? So if you wanted to know the molecular mass of... You wanted to know how much does one molecule of Benzene mass? I don't want to say "weigh" because it should be independent of what planet you're on. So what is the mass of one molecule of Benzene? Well all you do is you add up the masses of the different constituents. So you have six carbons and six hydrogen. So let's do Benzene. You have six carbons and six hydrogens. So what's the mass of each carbon? So let's go back to the periodic table. Just to give this proper credit, I got this off of the Los Alamos National Laboratories website. So let's see, the atomic mass of carbon... The reason why I use this one instead of my previous one is my previous periodic table that I got off of Wikipedia only had atomic numbers. But now that we're actually dealing with talking about the mass composition of different atoms or different molecules, we're going to have to start looking at the atomic mass, right? Remember, the atomic mass, when you think about it in atomic mass units, it's just the number of protons plus the number of neutrons. So you have six protons in carbon and roughly six neutrons. Why is there this decimal? Because we said before, this is an average of all of the masses of the isotopes that you'll find of carbon. So there's a little bit of carbon 14 on the planet, very little, but most of the carbon is carbon 12. When you proportionally average them, you get 12.01. Let's say we're dealing with carbon 12 because it's the most common element. So carbon is 12 atomic mass units. And atomic mass units is a unit of mass and we'll talk about how small it is. It's a very very very small fraction of a gram or a kilogram and we'll talk about that probably in the next video. So carbon is 12 atomic mass units, what about hydrogen? We go to our periodic table, hydrogen is here in this dark blue and the atomic number of hydrogen is one, the atomic mass of hydrogen is 1.0008. So that tells us that most of the hydrogen on this planet has an atomic mass of one. Which tells us that it essentially has no neutrons, that hydrogen is kind of an interesting nucleus there where there's really just a proton sitting in that nucleus. So if you were to ionize hydrogen, if you were to turn it into a cation and take one of its electrons away, what are you left with? You just have a proton. A proton sitting by itself. Just a single proton really is no different than a hydrogen ion, and that to me is kind of interesting. Hydrogen is that simple, it's really just a proton. So hydrogen has an atomic mass of one. If it had any neutrons in it, it would have been an atomic mass of two. But hydrogen has an atomic mass of one. One atomic mass unit. So what is the mass of one molecule of Benzene? Well it's six times the carbon mass, so six times twelve, plus six times the mass of hydrogen. Six times twelve is 72 plus six times one, plus six is equal to 78. Now, what if someone said, "What percent of Benzene is carbon?" Ok, well this is the piece that's carbon right here. The carbon piece of benzene is 72 atomic mass units, that's carbon. So what percentage of Benzene is carbon? Well it's 72 over 78. Benzene is 92.3% carbon by mass. And of course, the remainder 7.7% is going to be hydrogen. Let's do that for a couple of these other guys down here. So let's say we wanted to know what is the mass of a molecule of water? Well we already know what the mass of a hydrogen is, it's one. Hydrogen is one atomic mass unit. Oxygen is 16. Notice that it's exactly sixteen. So most of the planet, you pretty much have in an oxygen atom eight protons and exactly eight neutrons. So you get an atomic mass of 16. So oxygen has an atomic mass of 16. So the atomic mass of the entire molecule, you have two hydrogens, so you have two times the mass of hydrogen, plus one oxygen, plus 16. So it equals 18 atomic mass units for water. Once again if you want to say what percent, by mass of water, is oxygen? It's 16 out of the 18, right? So 88.9% oxygen. So most of water is oxygen even though you have two hydrogens here for every one oxygen. Oxygen's mass is so much larger, it's 16 times larger. That most of water is oxygen. The next video I'm going to talk about how we go backwards. If someone gives you the composition, how can you get the empirical formula? On a side note, I was doing some research about metals. Because they're actually interesting about why some metals conduct more and some conduct less and some sites... When I first talked about the transition metals, they're back filling their D orbitals. I said "Hey, the periodic table that was in a Princeton review book that described these as metals and described these as transition metals..." I said "Hey, that's kind of not fair because I consider iron and copper and gold and silver to be as metallic as anything. Why should these be called 'transition metals' and these be called just 'regular metals'?" It actually turns out that a common name for these are poor metals. Because, to a large degree they're softer, they have lower melting points, so the intuition was right. When we think of metals, these are the metals I think of and when we think of metallic nature in a chemistry sense, we talked a lot about that. Who wants to donate their electrons the most? That's metallic nature. They're the guys down here. Then as you go to the top right, these want to donate their electrons the least. These are the most electronegative. They like electrons the most. So they actually have some of the worst metallic nature. So it actually makes sense to call them poor metals. There's some debate as to whether these should be called poor metals. If you look up a bunch of periodic tables, some will call these metals, some will call these poor metals. But I just wanted to throw that out there so that you're exposed to it. For me, it is a little more intuitive to call these poor metals because they have less metallic nature than the stuff down here. The alkali and the alkaline earth metals. Anyways, see you in the next video.