Summarize this video hello everybody welcome back uh today we're going to be covering subunit 1.4 which is the composition of mixtures so let's get started so the first thing this is very very general that the first thing we need to talk about is just you know what is the definition of matter so anything occupying space anything that has volume and also has mass and typically we break the big heading of matter into two categories pure substances and mixtures so what's in that category of pure substances just two things compounds and elements okay elements of course you find on your periodic table compounds would be chemical combinations of elements compounds of course are things that can only be broken down by chemical processes you can't separate a compound into its individual elements through physical means has to be through chemical reactions so pure substances is one branch under matter the other branch is mixtures and you'll remember back from chem 1 that there are two types homogeneous and heterogeneous okay homogeneous another term that you'll see that that means the same thing as a homogeneous mixture is the term solution okay that is typically something dissolved in something else and that's a that's a whole other unit that we'll cover later in the year solution chemistry but in general homogeneous mixtures mean things that are evenly mixed uniform composition a heterogeneous would be still a physical mixture of things but it's not evenly mixed that there is variable composition so one part of the mixture is going to have a different composition than another part of it all right so what sorts of questions do we see about mixtures on the ap exam well they're they're what we call mixture analysis questions and i'm going to go through two types with you and here is an example of the first type okay so it says we have a 3.6 gram sample of a mixture of lithium fluoride and potassium fluoride okay so let me just pause there for a second so each one of those is a compound lithium fluoride is a pure substance a compound potassium fluoride is also a compound when you physically mix them together now it's a mixture but just to be clear lithium fluoride and potassium fluoride those two compounds are not chemically bonded to one another they're just physically mixed together so this mixture is found to contain 0.18 grams of the pure element lithium what percentage of the sample is made up of lithium fluoride okay so this is a common kind of question that you'd see on the ap exam and i'm gonna start with this point .18 grams of lithium and they don't want to know what percentage of the sample is made up of lithium they want to know what percent is made up of the whole compound lithium fluoride so i've got to somehow get to grams of lithium fluoride as we saw two lessons ago we've always got to go through moles so we're going to just take this step by step so the first thing i'm going to do is get out of grams of lithium and get 2 moles of lithium that number 6.9 that's coming off the periodic table i'm not ready to get to grams of lithium fluoride yet still have to go through moles okay now this step right here is a step that some of you will have done in chem 1 and others of you may not you can go mole to mole from a whole compound to a element or an element to a whole compound what you want to ask yourself in this situation is which thing is bigger the element or the compound of course the compound is bigger so i'm always going to put a 1 next to the whole compound now i'm going to ask you how many lithiums are in lithium fluoride well just one so these are in a one to one ratio if however and this is ridiculous but just pretend with me let's say there was a subscript of a 2 right there like li2 f then i'd have a 2 right there okay but i don't so i'm going to erase that okay now remember the objective here is to get to grams of lithium fluoride and now i can do that just extend that line just a little bit this is the molar mass off the periodic table and i'm going to plug all that in multiply across the top divide by any number on the bottom and i get this okay so that is the mass of lithium fluoride in this mixture well i know that the total mass of the whole mixture is 3.6 grams and they want to know what percent of the mixture is made up of lithium fluoride so just like any other percentage part over whole times 100 and i'm gonna round this to two significant figures since both numbers printed in the original problem have two so this mixture is only 19 lithium fluoride okay which means all of the rest of it must be made up of potassium fluoride okay so that's just one type of problem that you could see you know you're given a mixture of something and you have to determine the percentage of one part of that mixture here's another part another excuse me another type of problem you might see and i'm going to work this one out as well i'm going to work it out on a blank screen so let's look at it it says you have a solid that you know is mostly sodium chloride you suspect however that it may contain some sodium iodide potassium chloride or maybe lithium chloride so you suspect that it's got some kind of impurity in it when you analyze the sample you see that it contains 72 percent chlorine by mass is the sample pure sodium chloride if not what else does it likely have in it all right well what i would suggest that we do first is figure out what percent chlorine pure sodium chloride has so that way we can answer the question is the sample pure nacl if we get 72 then we can say yeah this sample is pure it has no um nothing contaminating it so let me get to a blank screen here and we're going to look at sodium chloride okay and we're going to determine the percent of chlorine within sodium chloride so there's one chlorine within nacl over the total molar mass one sodium plus one chlorine that has a molar mass of 58.5 and let's see what this gives us okay so if this sample was pure sodium chloride when we run an analysis we should have gotten 61 percent chlorine but let's come back to the problem we didn't it's got 72 percent chlorine so right off the bat is the sample pure nacl no it's not it's got something else in there and here's the clue guys if you let's come back here it should be 61 but it's some percentage higher so whatever is being mixed in there must have a chlorine percentage higher than the 61 percent because if it's averaging out to 72 percent then there must be something in there that has a chlorine percentage higher that's moving that average above 61. so let's look at our our candidates here all right let's come back here so here was one of the other possibilities of a contaminant okay now this is not a trick question what percent of chlorine what percent of this compound is made of chlorine zero sodium iodide does not have any chlorine in it so guys if you think about it if you were to you know average these two numbers it's going to be a percentage between 0 and 61. when i know that the percentage was 72 okay it's a higher number than 61. so nai is out that's not my contaminant let's try this one now this does have chlorine in it okay so it could be the contaminant let's see what its percentage of chlorine is so kcl also only has one chlorine 35.5 that's the molar mass of chlorine off the periodic table here's the molar mass of the entire compound and when i calculate it i get this okay now let's ask ourselves could could kcl be the contaminant no it couldn't because again look at those two numbers if i were to average them in some way it's going to end up somewhere between 48 and 61 percent that's not where the problem said that we were so process of elimination i know that lithium chloride must be my answer but i just want to prove it to you here's the percent of chlorine within lithium chloride and it comes out to be 84 okay now think about it if we were not necessarily an even average but if we averaged those two percentages we would get a number somewhere between 61 and 84 because it may not be a perfectly 50 50 split between these two compounds does that match the data that we were given yeah so i know this isn't it lithium chloride must be my contaminant because it has a percent of chlorine higher than the pure nacl and that's what the data was showing us so those are the two types of mixture problems that you will typically see sometimes they're in free response sometimes they're in multiple choice it just depends but those are the two most common to show up on the ap exam so i hope you have learned something and i look forward to seeing you all next time