in this first part of my final exam review I'll be helping a youtuber Selena Sanchez prepare for her final and understand stoichiometry now some of these questions are part of my story geometry notes that has each one of these types of questions going from grams to grams liters to grams and so much more so head over to Melissa Maribel comm or click the link in the description box to buy the notes and follow along alright you're ready to do some stoichiometry so these four conversion factors how I want you to think of stoichiometry and these these board problems they're kind of like puzzles and the puzzle pieces are these four main conversion factors so these conversion factors are going to tell you how to connect everything and how to you know how to move from one unit to a completely different unit so the first one that I want you to write down is molar mass so we know but I'm still gonna have you write it down that molar mass allows you to go from grams to moles or vice versa so literally just writing you know a double arrow if that makes sense to you or if not then grams to moles and moles to grams that's when you know I have to use the molar mass the next one was whenever you have to change the compound that's when you're gonna do a multiple ratio so because it goes from one you know moles of one compound to a completely different compound so these are just hints like so when you know what to do next and then we know a multiple ratio comes from a balanced equation the next one is Avogadro's number so this one this one seemed like you were ok with but we're still gonna write it down just in case that one goes from moles to any one of these units right atoms molecules particles formula units and vice versa so these can still keep going back and forth and the last one that you will potentially see is known as STP or liters at STP all right standard temperature and pressure mm-hmm and then and I'll go over each one but this was where our moles equals twenty two point four liters and what we'll go from there so I'll explain it in a second but these are the forming conversion factors that I want you to know because are the ones that are gonna help you fill in the gaps of oh wait I'm stuck I don't know all right so you're pulling from one of these conversion factors so for molar mass how do we fill up molar mass the masses of each one and they just Adam yeah I think your finest molar mass so this is where I had like let's say if you know what one we use molar mass we're going from grams to moles or vice versa moles to grams and just this is just a representation of how we always want to line the unit so they cancel right moles and moles and so on I think you're ok with that now multiple ratio let's talk about that one so a multiple ratio as we said before we always use it when we're trying to change the compound and this is this is once again going from moles of one compound to a different one and I have that here and we always align our units across so they can cancel it you're gonna keep hearing me say that and then this is going to be where you're going to find a mole to mole ratio so there's actually two different places that you're gonna find a multiple ratio the first one is going to be any coefficients that are in our balanced equation and as I mentioned if if there is nothing in front then it's going to be a 1 all right so if there's no coefficient there assume that it's one the last place that we're gonna find a multiple ratio is let's say they just gave you the compound so if they just gave you the compound I know it's going to be the subscripts in my compound so like how many oxygens do we have oh well there's three times two so six you know that would be six moles of oxygen how many moles of nitrogen do we have only two moles does that make sense mm-hmm so then that those are the two different places we can find a multiple ratio that being said why don't you try this one go ahead and pause the video and try this question out if a multiple ratio allows us to go from this compound to this compound right right now you only have the starting compound you don't have the ending compound yeah so that's what you need so you cancelled out the moles of nai right now you need the moles of what you're finding which is i2 so how many moles of i2 do we have one Sami sense yeah okay so these two cancel right mm-hmm not all stoichiometry questions are going to take like two conversion factors or even three conversion factors some might even just take one so and how you know that by the way is well this is just going from moles to moles right it's not going from grams to grams or it's not going from grams to molecules this hint is kind of like already telling you to use multiple ratio since you already have the two units of moles are the same it's just the compound that needs to be changed so same thing I want you to just write what are you given what are you finding set it up and then we'll go over it again give this question a try stoichiometry is just a way to cancel out units to get to the unit that you want but that's all stoichiometry is all right so so yes like you started off with you're given right yeah and what you're finding is where you end so that's why you're seeing what you start is literally where you start what you end with is always on top and then the conversion factor in between is most likely going to be on the denominator because that's what's cancelling out so the whole point of this we just want to cancel out units to get to the unit that we want okay let's do it just just in case just in case try this question out as Selena's working on it you should be too so pause the video and try this out on your own and we'll go over it together where I always want you to start off with is what are you given like what is this question give you one hundred and seventy four grams of co whatever carbon monoxide correct so see ya so then that automatically tells me where to start like since that's the only information that's given right that's your starting point what is this question asking you to find molecules of oxygen perfect so and then remember that oxygen is never gonna be by itself because it's known as the diatomic molecule meaning it's gonna exist in pairs that's all it means it means there's a two subscript okay that makes sense so oxygen is always going to exist in pairs so it's never just gonna be oh it has to be o2 so then I know okay well I'm finding molecules of o2 so right then and there you're given is gonna act as your starting point and your finding is gonna act as where you went now we just have to figure out the chunks in between mm-hmm so if we want to get from grams of carbon monoxide to then a completely different molecule what allows us actually what should we do first if we know for sure we're starting off with grams of carbon monoxide what should we convert to first molar mass we're gonna use molar mass beautiful the four different conversion factors that I was talking about all of them end up equaling moles so if at some point you have no idea where to start convert to moles and you're pretty much like 99% guaranteed that it's gonna be okay okay so if you have no idea where to start start with your given convert that to moles and then keep going and see where else you can go with it so in this case we are going to use the molar mass of carbon monoxide to go from grams of co2 then moles of CO so that's what we're gonna do next we're gonna that's our first step we're going from grams of CO 2 moles of CO and then from there we we now have moles of carbon monoxide or CO so where do you think we should go next to change because we want to change the compound all right we don't want Co we want o to the mall to mole ratio beautiful yes yes I have confidence yeah you know more than then then you're showing I know you or so moles of o2 you are gonna use that based on our multiple ratio so what is our multiple ratio like where do we get that information the sides is the yes correct the balance equation perfect so then I know that so far we started off with our given right and then we would find our molar mass of carbon monoxide just adding carbons mass plus oxygens mass and then those two would then cancel and now we have we're at moles of CO and the next we're gonna be using the multiple ratio so what should be on the bottom and what should be on top based on our multiple ratio the co should be on the bottom perfect how many moles of CO do we have - what about 402 I don't know so 400 - there's nothing in front if there's nothing in front assume there's a 1 mm-hmm so then there would just be one on top okay this is making sense yeah so then now that we're finally at moles of o2 well we got to the compound that we wanted right Oh - or the molecule that we want to know - we don't have the correct unit though we have moles what you want molecules so what allows us to go from moles to molecules Avogadro's number beautiful so then from there I know Avogadro's number is next you ever avocados number 6.02 times 10 to the 23rd beautiful that's what's next all right so then that allows us to get two molecules of o2 which is what we wanted from there of course we just have to multiply straight across and divide by these two moles fight go to this mm-hmm and then that'll give us our molecules okay questions so far No okay I'm gonna have you try one okay pause the video and try this out I don't know what to do okay so what was your first step that you did twelve point five grams of any 2o times something okay and then what are we trying to convert to grams sodium correct we said that if you don't know what to do first what should you convert you um moles moles how do we convert to moles how do we go from grams to moles the molar mass perfect so then what do we need that that step it's smaller mass exactly did you have your P our periodic table if not I can tell you the masses of Na and it has a molar mass of twenty two point nine nine so 44 plus sixteen yes whatever that is keep going with that I literally want you to write it down okay sixty and then it was a forty five point nine eight to twenty two times twenty two point nine nine times two twenty two point nine eight okay so sixty one point nine six it should have been nine eight sorry so the the NA mass of that was twenty two point nine nine I don't know if you heard me say ninety eight that was for something else mm-hmm so we should have gotten sixty one point nine eight okay keep setting that up see what cancels and then keep going from there what cancelled out the grams of any two Oh perfect so then we're at moles of any - OH mm-hmm but do we want do we want that compound no how do we change it I don't know okay so to change a compound we always do a multiple ratio mm-hmm so that's what's next so whenever we get it down to moles and then now we want to switch the compound you're gonna do a multiple ratio okay okay hmm so let's do multiple ratio alright so no you can keep going yet I'm just seeing if I have I don't want to put the next part what do we get for our multiple ratio mmm I don't know what I should put on the top or the bottom so look at the previous unit that you had so always make sure you align the same units across so they can cancel so if on top previously you had the moles of any two Oh mm-hmm then that should now be on the bottom right because we want those two to cancel d-rose I know so you have this so far right these two cancelled yeah you had your moles of an N - oh so we always want these two to be across from each other so they can cancel so then this tells us the moles of any two Oh should go on the bottom mm-hm and then what what we want to get to next should be on top which should have been our moles of any mm-hmm that part confused me this part yeah did it make sense that why these moles and moles have to be where they are currently yeah what part confuse you like and why like I never would have liked if I was doing this on my own I would have got stuck here and just like called it quits because I didn't wouldn't have known to put the two moles of whatever like I didn't understand were the two or the four came from but like I get it now but that's a good thing you get it now yeah hopefully okay I'm gonna be explain it just and we're gonna have to do plenty of these by the way so the next question we're gonna keep doing this and then I'm gonna have you try again plus at the end of course I'm gonna give you practice problems so this is gonna be a huge chunk of it I tell you that because you've probably seen on your exams already but stoichiometry is like the heart of chemistry mm-hmm so I'm gonna re explain this again all right just so we can really get this this going whenever we don't know where to start start with your given value and convert it to moles right that we know next whenever we're trying to change the compound we do in waltz mole ratio mm-hmm good with that and then of course if we then want to go back which is our last step here was if we want to go back to moles to grams you're still gonna use the molar mass so I'm gonna keep going with this part we were good with the molar mass it was the multiple ratio that was that where we kind of stumbled and then the reason why I knew it was a multiple ratio was because our unit was already in moles right and where else can I go I can't just instantly I don't want to jump back down to go to grams because that would that wouldn't give me what I want so multiple ratio comes from our balanced equation right the four moles of NA come comes from this balanced equation that was our reactant our product was then two moles of any two oh that one on the bottom always a line are you so they can cancel out now that we did that we're on our third step which is going from the moles of na to the grams of NA and we're going to use the molar mass of sodium from there I'm going to put that on top and then the reason why I place one mole on the bottom is once again so it can cancel out so that's that's how I want you to see this is if you're not sure what to do next or how to align the units if you've gotten at least to the first step it tells you what unit comes next mm-hmm right it tells you oh this has to be aligned on the opposite place do we see that pattern yeah okay you do great if you found this review helpful then you'll really enjoy my complete guide to stoichiometry which has even more examples and comment test questions head over to Melissa Maribel comm or click the link in the description box to buy the notes and I'll see you in the next video covering limiting reactants