total mass of all substances is conserved in a chemical reaction that must mean the atoms that go in must come out so we must balance equations to that end we already know about relative atomic mass but if it's a compound we can add these up to give the relative formula mass we just add up the individual Rams so CO2 is 12 plus 2 lots of 16 so that's 44 some reactions produce a gas product which if it leaves the reaction vessel will result in a seeming decrease in mass of the reactants a mole is just a specific number of atoms or molecules but we don't really need to know the number it's just a way of comparing amounts of substances as we can't deal in individual numbers of atoms or molecules if your foundation you don't need to deal in moles by the way if you have as many grams of a substance as its relative atomic or formula mass you have one mole so one mole of carbon has a mass of 12 G that means we calculate the number of moles of something we have like this moles equals g over Rams where Ram is short for relative atomic mass but it also could be relative formula mass this is an equation worth remembering let's take our methane combustion reaction from earlier like we said in order to balance this we'd need two oxygen molecules per one molecule of methane this is also true for moles too then we'd need double the moles of oxygen to methane so here's how a question could go how many grams of water would be made if 64 GS of methane reacted completely with oxygen we need to get from the mass of one thing to the mass of another so we use moles as the middleman the process is this Mass moles moles Mass we switch from one to the other at the halfway mark So a mass of 64 G of methane how many moles is that moles equals g over RS so that's 64 ID 16 that's 4 moles of methane but look there's no number in front of the methane but there is a two in front of the water which means we must have double the moles of water so that's 8 moles by the way we can say that the stochiometry is 1 to 2 that just means the ratio of moles of one substance to another in a reaction so what we have to do then is turn that back into Mass using our equation by rearranging it put it into a triangle if you have to and cover up Mass g equals moles time Rams so that's 8 moles * water's Ram of 18 that's 144 G of water made you could also be given the mass in kilogram or even tons the great thing is is that because this is all relative we can just put those masses into our equation instead of grams and so long as you stick with that unit for the whole question you'll still end up with the right answer of course we can also use moles to predict how much of a reactant we would need in a reaction as you can see we need two moles of oxygen to every one mole of methane if we had that one mole of methane but only one mole of oxygen that means that not all of the methane would react some would be left behind we say that the oxygen is the limiting reactant in this case it ran out first the concentration of solutions can be given in G per decim cubed where a decim cubed is 1,00 cm cubed but it's often useful to convert this into moles per decimet cubed instead if one mole of HCL is dissolved in 1 decim cubed of water we've made hydrochloric acid at a concentration of 1 mole per decim cubed sometimes we shorten this to just one Moler triple only now until the next topic chemical changes in many reactions we want to make as much product as possible more often than not though there will be some reactants Left Behind over at the end like we know for example if a reaction is reversible like the harbor process test to make ammonia more about that in paper to you'll always end up with hydrogen and nitrogen at the end in this case when it's reached equilibrium percentage yield merely tells you how much product is actually made compared to how much you could have made in theory had all the reactants reacted for example if you start with 20 G of reactants here but only end up with 10 G of ammonia the percentage yield is 50% you must be given the actual masses involved in questions on this so you can't predict what the yield would be just from the equation atom economy on the other hand tells you how much of a desired product you get out of a reaction compared to the mass of the reactants that went in you use relative atomic or formula masses to do this I like to think of atom economy as efficiency of mass we calculate it like this the ram of desired product divided by the total Ram of reactants Times by 100 back to the methane reaction sometimes this is done in green houses to make CO2 for the plants it's an incredibly important gas necessary for life to thrive you see the ram of CO2 is 44 so that goes on top of our equation now we could calculate the ram of the reactants but there's a Nifty shortcut we can take here because this is also the same as the ram of all of the products due to conservation of mass as we know so we might as well use that seeing that we've already got the RAM for one product add on two lots of 18 so that's 44 divided by the total of 80 * 100 that's 55% one mole of any gas takes up a volume of 24 decim cubed regardless of its relative mass this is true for RTP room temperature and pressure that's 20° C and a pressure of one atmosphere you must be able to convert moles to volume and back by multiplying or dividing by 24 so I hope you found that helpful leave a like if you did and pop any questions or comments below and I'll see you in the next video