A Level Chemistry – The Mole Concept

hey guys welcome back the next series of videos are all about looking at some fundamental ways in which we quantify amounts of different substances in chemistry this branch of chemistry is called quantitative chemistry but no need for fancy words here it literally is just looking at and calculating amounts of stuff now there's literally no way we can look at the amount of something in chemistry and not come across the infamous mole it's actually really great that you're watching this video because the mole is probably the most important thing you'll learn for a little while okay so to get in the spirit a bit I want you to imagine having two separate collections of basketballs and golf balls weighing ten kilos each now think about the difference in the amount of balls between the two you don't need to be a dreamiest to know that they'll definitely be more golf balls than basketballs 218 compared to 16 in fact and this is the case even though the total masses are equal to each other you can play a lot more games of golf than you can basketball I see you in you don't lose heaps of balls in the water like I did when I tried so why did I bring up this example well because this exact same idea exists in chemistry when we deal with particles of different sizes we're easy able to determine mass by weighing something but measuring an amount of substance by mass doesn't directly give us the amount of particles in that substance we need to know amounts of particles to predict reactions and this is where moles come in to link the mass of a substance with the number of particles in it you can check out the attached syllabus dot points on the video page to see where all of these content comes from so we'll firstly look at what moles actually are and from this we'll do a couple of conversions between an actual number of particles and a number of moles of that particle we have will then put on our thinking caps and actually try to make sense of the theory surrounding them all concept and how this ties in to the molar masses of different substances and finally we'll link this all really briefly into the idea of stoichiometry which sounds really complex but trust me it isn't so with all that to do let's get cracking with an introduction to the concept of moles okay whenever we measure things we use units to help us organize our quantities for example if we wanted to measure the length of this video we'd probably use seconds or minutes as our units of time as these are pretty widely known and convenient but if I were to tell you that the time of recording this video I was born eleven million nine hundred and eighty seven thousand six hundred and forty minutes ago you'd think it was a right weirdo and that's because when you're asking the age of somebody you typically expect the answer to be in years now I'll leave it to you to figure out how old I am in years but chemists are always dealing in numbers of atoms and molecules and if you think about it this could get pretty tricky as these are so tiny and there are so many substances we want to measure even a small amount of matter contains trillions of atoms so it makes sense that we would use a large number to help us count the number of atoms or molecules in a substance that specific number 6.022 times 10 to power 23 is known as Avogadro's constant it gives us the number of particles in a mole don't worry about needing to remember this it's going to be given to you on your formula sheet so in other words no matter what substance you're talking about we say a mole of it is 6.022 times 10 to the 23 particles of it so now that we know what a mole is it makes sense that we should be able to convert between an actual number of particles a number of moles we know that there is avocados number of particles in one mole so converting from number of moles which is written as a lowercase n to particle number we simply multiply by Avogadro's number which is given by this symbol and it's made pretty obvious by this little a here of course the opposite direction applies to converting from number of particles to number of moles requires division by Avogadro's constant okay great so let's do a couple of examples there's six moles of a substance how many particles is this well 6 multiplied by Avogadro's constant gives us 3 point 6 1 3 times 10 to the power 24 all right if I have 11 times 10 to the power 23 atoms how many moles do I have Wow 11 times 10 to power 23 divided by Avogadro's constant equals one point eight three moles ah easy-peasy so I can almost hear you guys saying yeah it makes sense that we use a really big number to count molecules and atoms but 6.022 times 10 to the 23 seems a bit random down there well you're right it is a pretty weird number but it's like that because it's been specifically designed by chemists to be the most useful number possible one mole is defined as the exact number of carbon-12 atoms weighing 12 grams carbon-12 is chosen as the reference point for most things to do with comparing elements and so this is no different this is so useful since we can use this to relate the mass of a substance to the amount of moles the atomic weights or relative atomic mass on the periodic table is in atomic mass units amu and tells us the mass of one mole of a substance so looking at our atomic weights we know that hydrogen has an atomic weight of one point zero one atomic mass units meaning that one mole of hydrogen atoms will weigh one point zero one grams in other words one point zero one grams of hydrogen has six point zero two two times ten to the power 23 particles in it furthermore oxygen has a relative atomic weight of sixteen point zero zero eight amah class units meaning one mole of oxygen atoms will weigh 16 point zero zero grams and so on just a little point to notice here hydrogen and oxygen typically exist as diatomic molecules which is just a molecule made of two atoms at the same element so if we were asked to find the molar masses of hydrogen or oxygen gas we'd need to multiply these values by two if you guys were at all unsure about how we get these numbers for our relative atomic weights that's all good just go back and check out the past videos on relative mass so to tell the mass of one mole of an atom which is called the molar mass it's literally as easy as reading the atomic weight of the corresponding atom off the periodic table and writing this in terms of grams again if this is seeming totally new to you and you want to check it out in more detail head back to the video called relative math part two we are then able to use this same idea when talking about molecules as these are just combinations of atoms for example one molecule of water is made up of two hydrogen's and one oxygen meaning its molecular weight is equal to 2 times the atomic weight of hydrogen plus the atomic weight of oxygen which equals 18 point o2 that means that one mole of water molecules will weigh eighteen point zero two grams okay I think it's important here to clear up a little bit of terminology that can be a bit confusing the difference between molecular weight and molar mass molecular weight is simply the combined atomic masses of all the atoms in a molecule written as number this is obviously going to be a tiny mass and so it's in atomic mass units but the key thing is that the molecular weight is equal to the mass in grams of a mole of that molecule so to convert this into a molar mass M R which is the mass of one mole of a particular molecule we simply write in the unit's grams per mole after it's really that simple using our molar mass will become really useful in our master mole calculations which we'll dive into in another video so definitely tune in for that so what on earth two moles have to do with our chemical reactions well this is where we introduce the term stoichiometry simply per stoichiometry is the idea of using ratios in chemical equations to calculate amounts of substances being used up or produced in a reaction so just looking at our reaction to produce water before we thought of the numbers here is referring to the number of atoms involved in the reaction but of course since moles are just a way of counting atoms the same ratio will apply to moles as the amount of atoms so we can now read into this is telling us that for every two moles of hydrogen gas and one mole of oxygen that react to moles of water molecules are formed these are known as the molar ratios of this equation we can then use the same ratio to tell us that one mole of hydrogen gas and half a mole of oxygen gas react to produce one mole of water and so on all right guys that's really about it for moles so I'm gonna put the important points to take away from this video up on screen now just one other thing to know as we progress through the course and do even more calculations the concept of moles will continually pop up so it's not as though this is a one-time learn type of thing with some practice these concepts should become a pretty effortless part of some more difficult questions anyway I'll catch you guys in the next one [Music] [Music] you [Music]

Channel: Atomi

Views: 75,874

Keywords: Atomi, a levels, chemistry, the mole concept, gcse, mass and particle numbers, the relationship

Id: jCynTWTwjpw

Length: 9min 59sec (599 seconds)

Published: Tue Jul 02 2019