 Hi, I'm Zor. Welcome to Ineasur Education. We continue talking about units of measurement of different physical substances and today we will talk about some, I would say, specific unit. It's kind of different for different substances. It's called a mole. M-O-L-E mole. It comes from molecules, obviously. Okay. This lecture is part of the course called Physics for Teens, presented on Unisor.com. This is a website. There is a prerequisite course, Mass for Teens, on the same website. The website is totally free. There are no advertisements, so it's all pure knowledge, open for everybody. Every lecture on this website is presented with, it's a video recorded, obviously, and it's presented with notes. Notes are organized basically like a textbook. So you have a video presentation and textual presentation of the same material. So I suggest you to use the website for these lectures. So if you found it by accident or by search engine, whatever, somewhere on YouTube or somewhere else, well, it's good. It's just one particular lecture, but on the website you have this Unisor.com, you have the whole course, basically, and some other materials. There are some functionality of the website, which you might be interested in. Okay, so let's talk about amount of substance. How do we measure certain amount of substance, which is convenient for certain purposes? And here is, I think, a very good example. Consider, we have a rocket, which works on hydrogen. So it burns hydrogen. Now it's in space, so to burn hydrogen, we need oxygen, right? So there is a chemical reaction between hydrogen and oxygen. They combine together when hydrogen is burning and the burning itself is basically a combination of hydrogen and oxygen combined together to become a molecule of water, well, the vapors of water, which which go out, obviously, and that's how the rocket moves. So what the reaction actually, a chemical reaction, is. Now, as we know, water is the molecule of water contains two atoms of hydrogen and atom of oxygen. Structurally, it works like this. This is a molecule of water. So we need two atoms of hydrogen per one atom of oxygen, right? Now, hydrogen is molecular. Molecule is basically two atoms together. Oxygen is also molecules. Two atoms of oxygen are combined together into one molecule. So to create water, we need two atoms per one. So if we are talking about molecules, we need two molecules of hydrogen and that would give me two molecules of water, right? Four atoms of hydrogen, four atoms of hydrogen, two atoms of oxygen, two atoms of oxygen. So this is the reaction, basically, chemical reaction of burning of hydrogen. Now, since it's a rocket, we don't want to take any extra hydrogen or extra oxygen. We need exactly what we need and we need all hydrogen to burn, consuming all oxygen, so there is nothing left. I mean, it's natural. We don't want any excess weight or anything. So how can we have the exact necessary amount of hydrogen and oxygen so they will burn and everything will be consumed? Well, in this particular equation, we see that we need twice as many molecules of hydrogen as oxygen. How can we achieve this type of? How do we measure certain amount of hydrogen so it contains exactly twice as many molecules as amount of oxygen? Well, the good approach probably is the following. Atom of hydrogen, of element hydrogen, not molecule, is one proton and no neutron, so the atomic mass is one. Oxygen has eight protons and eight neutrons and that's coming into atomic mass of 16. Well, if atomic mass of oxygen is 16 and atomic mass of hydrogen is one, most likely if we will take 16 gram of oxygen, it will contain exactly the same amount of number of molecules as one gram of hydrogen. So if one atom is 16 times heavier, then 16 gram of oxygen would contain the same number of atoms or molecules, it doesn't matter in this case, as the number of molecules of hydrogen. Well, because both molecules contain two atoms. So 16 gram will contain the same number of atoms as one gram of this and since the same number of atoms will be the same number of molecules because two atoms per molecule. So this is an approach which can be taken. Knowing atomic weight of every element, we can basically have the amount of the corresponding substance proportional to this atomic weight. So in this particular case, the proportion is one to 16, right? Now, what I can just generally say that if you have an element, x, element x, it has molecular, it doesn't matter, it's an elementary kind of, if it's an element or it's a complicated molecule, it doesn't really matter. All atoms which are combined into this molecule together have combined atomic mass Nx. Now, if I will take another substance, molecules of type Y and they have N, Y, combined molecular mass or molecular weight, which is basically number of protons and neutrons inside all the atoms, which constitute this molecule. And this is total number of protons and neutrons, which combined together constitute this molecule. We right now don't really pay attention to electrons because they're very small and the gain protons and neutrons are not exactly of the same mass. There are some deviations, but let's right now simplify the whole picture approximately. If we will take this number of gram, gram, or kilogram, doesn't really matter. And this number of gram of this substance, the number of molecules would be approximately the same. That's what I want to say. That's what's the most important part of it. The number of molecules, actually, is proportional, well, if the weight or mass is proportional to molecular weight of these two substances, then the number of molecules should be approximately the same. Now, as an example, what is molecular mass of what do they have? Hydraulic acid. Hydraulic acid is H, chlorine. So the, this is the molecule. The atom of hydrogen has one proton. Atom of chlorine has 17 protons and 18 neutrons. So all together one proton from hydrogen, 17 protons of chlorine and 18 neutrons of chlorine. That gives me 36. So 36 grams of hydraulic acid would contain exactly the same number of molecules as, let's say, two grams of molecular hydrogen or 32 grams, 16 plus 16, of molecular oxygen, the same number of molecules. So 36 grams of this, 32 grams of this, 2 grams of this, they're all the same number of molecules. Because these masses are proportional to molecular weight or molecular mass of the corresponding molecules. So what is this number? Well, if you take grams, grams is specifically grams. The number of grams exactly equal to the molecular weight. Let's say, 36 in this case, 32 in this or 2 in this. We will have certain number of molecules and it was calculated and it's called Avogadro number. Avogadro. And it's equal to 6.022. 1, 4, 0, 7, 6 times 10 to the 23rd grade. So approximately this number of molecules of hydrogen are in 2 grams of hydrogen. Or the same number of molecules of oxygen contains in 32 grams of oxygen. Or the same number of molecules of hydrochloric acid in 36 grams of hydrolic acid. The same number of molecules contains here. So this gives me sufficient knowledge about how to, for instance, put how much oxygen and hydrogen should I put on a rocket since I know the number of molecules. So I know it's supposed to be like 1 to 16. But I need two molecules. So I probably should take like four of this per 32 of these and that would be fine. Okay. Now, this is kind of approximation. Well, first of all, mole is so mole is in case of oxygen, mole is 32 grams and mole is in case of hydrogen is 2. It's not exactly the unit of measurement because it's different for different substances. It's 2 for hydrogen, 32 for oxygen, etc. But anyway, it's called a unit of measurement of substance, amount of substance. It's used obviously in chemistry quite widely. So and that's why it's basically part of the this universal system of physical constants, physical units of measurements, which we are talking about called C, right? Now, in 2019 people decided that this is not exactly kind of a nice way to define. But first of all, these numbers are approximate. The precise numbers are, if we will take into consideration, what is the unit of atomic mass? Unit of atomic mass is not just weight of one proton. That's not example correct. Unit of atomic mass, mass we were talking about, it's one-twelfth of carbon-12 atom. Carbon-12 atom contains six protons and six neutrons. And six electrons. So whenever we will, you know, divide it by 12, we will have something which is not exactly the proton, not exactly neutron, it's something in between. And obviously, these numbers are approximate. More precise, for example, number four hydraulic acid would be not 36, but 36.458. So that's exactly the mole in grams, for example. That's the mole of hydraulic acid, because the molecular weight is 36.548 of atomic mass units, which are one-twelfth of carbon-12. It's kind of messy, a little bit messy. So what people have decided in 2019 physicists were responsible for C, for this system of unit of measurements, they've decided, okay, you know what guys, this number, Avogadro number, is constant. So this number of molecules of any substance is a mole of this substance by definition. And that's basically kind of how much it weighs, how much mass for instance, these molecules. Well, it will not be, for instance, for in case of hydrogen, this will not be exactly two grams, obviously. It will be two point something, but let's not talk to it what it exactly. But anyway, this number of molecules becomes a standard in C. And that's not only the molecules, basically it can be any kind of particles. I mean, so the the standards doesn't really specify that it's molecules, but obviously, in in for most practical purposes, we are talking about this exact number of molecules constitutes one mole of any substance. And let's say in case of hydrogen, we can say that this number of molecules is a mole of hydrogen. This number of molecules of hydraulic acid is the mole of hydraulic acid. So that's what I meant when it's kind of different for different substances, different in grams, obviously, for different substances. But it's the same as number of molecules. So number of molecules is the standard, not grams which kind of approximates the number of molecules. It's number of molecules which becomes really a standard, a definition. Well, and that's it. Read the notes for this lecture. It's on unison.com. We have to go to physics for team course. There is a part of the course, which is called units of units in physics. And then the menu is base C units and among base C units, you will have the amount of substance as basically mole, which we were talking about. That's it. Thank you very much and good luck.