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# Molecular mass and molecular weight

## Video transcript

- [Voiceover] What I wanna do in this video is get a little bit familiar with the idea of molecular mass. Let's start off with a molecule that we're all quite familiar with. In fact, we are made up of this molecule or to a large degree, made up of this molecule. That is water and the molecular formula for water is H2O. Every water molecule has two hydrogen atoms and one oxygen atom. What I'm curious about is, what is the mass of a molecule of water? So, the way that you might think about doing that is, what's the mass of each of the hydrogens? You're gonna have two of those and then what's the mass of the oxygen? That would be a reasonable way to do it. So, what is a mass of the hydrogen? A hydrogen has a mass... We can go to a periodic table here, I've got one right over here. You see hydrogen, it has its atomic number one. That's the number of protons, that's what makes it hydrogen. Then down here you have this number. I think you can see, it's a little bit small probably, on your screen, it says, "1.0079". This right over here is the atomic weight of hydrogen. This is, if you take the various isotopes of hydrogen on the planet earth and you take the weighted average. So the ones that are more common, they are weighted higher. The weighted average of their atomic masses is 1.0079. Now for the sake of this first pass, we're just going to say, "It's roughly one atomic mass unit." and one way to think about it... This isn't precise and it's not how things are defined, but it is a useful way to think about atomic mass or mass on the atomic scale. Is that one neutron, or one proton have an atomic mass of approximately one atomic mass unit. In other videos we'll talk about why it's not defined this way and why it's based on the carbon-12 isotope and all of that. But this is a fairly useful thing that is helpful to think about, what is the atomic mass likely to be? What is it roughly? When you see that hydrogen, it has one proton. Clearly, that's what makes it hydrogen. It also has an atomic mass of one or roughly one, it says, "Well, it must not have a neutron." That is the case, the most common isotope of hydrogen on earth is just a proton with a electron. The electron also does contribute to mass but it's negligible compared to the proton. Most of this mass is essentially the proton. A little bit of the electron and also kind of the weighted average, when you think about the other isotopes of hydrogen that maybe might have a neutron that is involved with it. For the sake of this, we could say, "Hey, hydrogen, it's a proton with an electron "kind of buzzing around it." It is approximately one atomic mass unit, and then oxgen. Oxygen, same idea, we'll look at the periodic table. You could guess what its mass is close to. Oxygen, the most typical isotope of oxygen that you'll see on earth has eight protons. The eight protons are what makes it oxygen. Any oxygen has to have eight protons anywhere that you are in the universe, if it doesn't have eight protons it's not oxygen. It's gonna have eight protons and it's gonna have eight neutrons, that's the most typical isotope of oxygen. So you might guess that its atomic mass is 16 atomic mass units. You can see that your guess is pretty good. When we look at our atomic weight right over here. Which is the weighted average of the various isotopes of oxygen as found on earth. You get 15.999, which is pretty close to 16. Our approximation to 16 is pretty good. Then based on these numbers you would say that this H2O has an atomic mass of approximately... Well, two from the hydrogens, where did I get the two from? Each of these two hydrogens have an atomic mass of one. If you have two times one, it's just gonna be two atomic mass units. Then 16 from the oxygen. Two plus 16, which is going to get us... Let me do this in another color, since I've been using... It's going to give us 18 atomic mass units. Now, if you wanted to be more precise or if you wanted to say, "Well, I have this big bag "of water, I'm not looking exactly at one water molecule. "What is going to be, on average, "the mass of those water molecules?" It might be helpful to get a little it more precise. Then it is helpful, especially if you're talking about a large number of molecules and you really just wanna take the weighted average of all of those molecules. Then it makes sense to say, "Well, let's "use the atomic weight." So for hydrogen, the atomic weight was 1.0079. Which is, once again, 1.0079 atomic mass units. We call it atomic weight but it's really just the weighted average, it's not weight in kind of the physics sense of measuring a force. So, 1.0079 atomic mass units and the oxygen is is 15.999 atomic mass units and if you wanted to get a more precise number here, let's get a calculator out. I got my calculator. I'm gonna have two times 1.0079 is equal to, and then to that I'm gonna add, plus 15.999. Gets us to... Let's see, I should go no more than three decimal places to the right. Since I added this, if I don't wanna add precision here... If you wanna a review of that, you should look at the video on significant figures. Since I added something with just three decimals to to the right I shouldn't have more than three decimals in my answer so, 18.01, I'll round in the thousandths place, 015. So the real one is 18.015 atomic mass units. You could actually consider this the molecular weight. Because once again, we're using atomic weights. We're using weighted average and you could think of, maybe the weighted average of water molecules would be a little bit closer to this. These two numbers are very close. So we might call this molecular weight. Either way, these ideas are very closely related. When we're thinking on the atomic scale, when people use the word weight, they're not using it like a force like you use in physics that you would measure in newtons or pounds. They are essentially taking a weighted average of masses. So the molecular weight of water, 18.015 atomic mass units. If you wanted to just kinda be a little bit loosey goosey, you could say it's roughly 18 atomic mass units.