An Actually Good Explanation of Moles

the concept of moles in chemistry is famously difficult to explain and understand amor is an amount of something so you might say I have a mole of carbon dioxide or I have a mole of glucose and you can see why it's a difficult concept to explain if you just look at the definition of the mole so by definition a mole is six point zero two two one four zero seven six times ten to the 23 particles or things in other words if you have 602 sextillion 214 quintillion and 76 quadrillion things then you have a mole of those things so I guess you understand what a mole is now except like why is it that and how do I use it it's a really opaque definition we need to get to the bottom of the purpose of the mole and actually I think moles fall into a category of things that are difficult to explain and understand until you first look at the problem that that thing solves and then it becomes really easy the offside rule is another example maybe I explain the offside rule at the end of this video as well but for now we're gonna look at moles so by the end of this video you will really understand moles so what problem does the mole solve for chemists well chemists like reacting things together so maybe they've got some hydrogen over here and some fluorine over here they want to react them together to make hydrogen fluoride but they want to do it in such a way that there's nothing left over there's no residual hydrogen there's no residual fluorine how do you achieve that well you start by looking at the chemical equation and you can see from this a chemical equation here hydrogen plus foreign goes to hydrogen fluoride by the way this isn't a realistic chemical reaction this is a simplified view of the process that is instructive for this explanation but anyway you've got one of each thing and they join together to create a molecule that has one of each thing in it it's very simple so you need the exact same number of hydrogens and fluorines how do you do that you could count them out so you could count out a trillion hydrogen's and a trillion fluorines you know you've got the same amount of both there'd be nothing left over if you react them together but that's impractical so you could do what chemists always do which is to weigh out the different chemicals so you weigh out one gram of hydrogen and one gram of froy then you've got the same amount of both except that doesn't work because fluorine atoms are heavier than hydrogen atoms so if you've got one gram of each you'll have more hydrogen atoms than fluorine atoms so what you have to do is look up the relative masses of hydrogen and fluorine and when you do that you find that fluorine is 19 times heavier than hydrogen like if you were going to react single atoms together so you've got one atom of hydrogen one atom of fluorine the ratio of the masses in that chemical reaction would be 19 to 1 and so long as you keep that mass ratio the same you know you'll have the same number of hydrogens as you do fluorines so for example if you've got one gram of hydrogen you'll need 19 grams of fluorine or you could do two grams of hydrogen and 38 grams of fluorine or you could do 10 ounces of hydrogen and 190 ounces of fluorine so long as the ratio is 1 to 19 you know you'll have the same number of atoms in each pile so how do we know that an atom of fluorine is 19 times heavier than an atom of hydrogen well most of the mass of an atom is in the nucleus because the electrons are so incredibly light so we just need to look inside the nucleus and see what's there for hydrogen the nucleus is just a proton one proton so we could say that the mass of hydrogen is equivalent to one proton or to use the proper jargon we talk about atomic mass units so we would say that hydrogen has an atomic mass of one I'm oversimplifying slightly which I'll clear up at the end but go with it for now if you look inside the nucleus of fluorine it has nine protons and ten neutrons so it has an atomic mass of 19 well so far our imaginary chemist hasn't come up against anything particularly challenging that's for two reasons the first reason is that the chemical reaction is really simple you've got two atoms going in your one molecule coming and that molecule is made of one of each of the atoms the second reason is that one of the chemicals going in is hydrogen and that has an atomic mass of 1 which makes the mathematics really easy in reality chemical reactions are usually much more complicated than that for example the chemicals going in won't be atoms there'll be molecules and the results of the chemical reaction might be more complicated like it'll have six of one thing in two of the other which makes the mathematics more complicated so let's imagine a slightly more complicated scenario this time let's look at beryllium oxide here's the chemical equation for it again this isn't how you make beryllium oxide it's a summary view it's still really simple but now the masses are more interesting so if we weren't to react a single atom of beryllium in a single atom of oxygen what would the ratio of the masses be well oxygen has an atomic mass of 16 beryllium has an atomic mass of 9 so the ratio is 16 to 9 which is why I call this chemical the wide-screen chemical so when you're reacting single atoms together the ratio of the masses is 16 to 9 and so long as you keep that mass ratio the same you'll have the same number of both atoms and you won't have anything left over after the chemical reaction takes place that's called a stoichiometric ratio by the way so a sensible choice might be to have 16 grams of oxygen and 9 grams of beryllium this logic works for molecules as well by the way so if the chemicals going into your reaction are molecules you just count up the number of protons and neutrons in the molecule and that gives you the atomic mass of the molecule and so chemistry proceeds in this way whenever a chemist wants to react and things together they work out the atomic mass of those things and they get that much of the stuff but in grams or kilograms or whatever or tenths of grams or tens of kilograms so you can imagine chemists having conversations like this how much oxygen have you got there well you know the atomic mass of oxygen well that but in grams mm-hmm how much hydrogen have you got there well you know the atomic mass of hydrogen you know that but in grams well actually double that I'm making water so for every oxygen you know you go to so that carries on for a number of years and then I imagine some clever chemist comes along and says do you know what instead of every time saying you know the atomic mass of X well that but in grams why don't we just say moles so you say I have a mole of sodium chloride I have a mole of oxygen and we all just agree that when you say that what you mean is you know the atomic mass of that thing well that but in grams so for example aluminium has an atomic mass of 27 so if you've got 27 grams of aluminium you have 1 mole of aluminium and it just makes everything a lot easier like you can look at any chemical equation and just go so I clearly need one mole of this three moles of this and I'll get two moles of this then when it comes to actually doing the chemical reaction you just look up the atomic masses of the chemicals going in you get that much of the chemicals in grams and you know you have the perfect ratio of chemicals you might be thinking at this point that the definition I just gave you of the mole is really different to the stated definition that I gave it at the start of the video with that really big number in it I'll get to that in a second but first I want to talk about a simplification that I made earlier I said that hydrogen has an atomic mass of 1 oxygen has an atomic mass of 16 that's not quite true for example hydrogen has an atomic mass of 1.008 oxygen has an atomic mass of 15 point nine nine nine they're all a little bit off from whole numbers the reason for that well there's two reasons the first is relativity we know from relativity that energy has mass equals mc-squared and the bonds between protons and neutrons contain energy and that energy has some mass the second reason is isotopes so hydrogen is usually one proton with an electron going around it that's the vast majority of hydrogen but occasionally you get hydrogen that where the nucleus is a proton and a neutron that's called deuterium there's a tiny amount of that occurring naturally on earth so if you want to say what the atomic mass of naturally-occurring hydrogen is you need to take into account that tiny amount of hydrogen that is actually deuterium that's not just true for hydrogen of course other atoms have different isotopes as well and because the atomic mass of atoms deviate slightly from just counting the number of protons and neutrons in the nucleus we need to be really careful to properly formally define atomic mass units so you could for example say okay by definition one proton on its own not bonded to anything else has an exact atomic mass of 1 and then everything else is based on that for some reason scientists decided not to do that they went with carbon instead maybe because it's easier to measure it's more abundant something like that so the formal definition of atomic mass units is one atomic mass unit is equal to 1/12 of the mass of carbon-12 where carbon-12 is the isotope of carbon that has 6 protons and neutrons in the nucleus which is why an older definition of the mole was the amount of atoms in 12 grams of the carbon isotope carbon-12 which arguably is even more confusing how does that relate to the modern definition with that huge number in it well if you define atomic mass units as 1/12 of the mass of carbon-12 you don't actually need to know the absolute mass of one of those atomic mass units you don't need to know what it is in grams or kilograms or anything like that you don't need that information you can carry on just comparing the atomic mass units of different chemicals and then weigh out moles of those things and proceed with your chemical reactions with confidence but as we became better scientists we were able to eventually work out the absolute mass of a proton or the absolute mass of an atomic mass unit and it's some tiny fraction of a gram and so from that point you can work out how many atomic mass units there are in a gram and that's the huge number it's called Avogadro's number and so now we've come full circle if you measure out a gram of something that has an atomic mass of 1 then you'll have an Avogadro's number of that thing generally the use of moles is confined to chemistry but you could use it for other things for example there is one micro mole of sand grains on the earth I have a draw in my house with half a mole of cables in it I said I'd also explain the offside rule and I have it's in a separate video it's a patreon exclusive link to my patreon page in the details I've been using brilliant org for a 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Channel: Steve Mould

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Length: 13min 36sec (816 seconds)

Published: Thu Jul 23 2020