summarize this video [Music] this video will talk about ap topic 1.4 which is compositions of mixtures so we'll talk about how you can use the proportions of the elements in a compound to actually determine the proportions of the compounds in the mixture so we know that matter can be divided into two categories pure substances and mixtures last video we talked a little bit more about pure substances this one's we're going to talk about how we can use ideas of pure substances to look at mixtures so we know that pure substances are one type of particle right they can be an atom they can be molecule they could be a formula unit but the composition of that particle is always fixed and that is because that is what makes the identity of that particle right but when we talk about mixtures mixtures contain two or more different types of particles right and in in mixtures the compositions can vary we often talk about molarity or concentration so molarity is the most common way we look at concentration but we could look at weight per weight so like how much mass of this thing versus the mass of this thing often times we talk about that in water so we maybe have 50 grams of this dissolved in 100 grams of water or we can do volume and volume so 40 milliliters of this and 100 milliliters of water oftentimes water is one of the components of our mixture okay um we can also talk about the um what is in the mixture based upon the proportions of the elements in that mixture because we know that there is a constant proportions of elements in a compound so therefore there should be a constant proportion of those elements in the mixture based upon the ratio of the compounds in the mixture okay it's a little indirect way of thinking about it but it still maintains that there's always a constant proportion of these okay so again we can know that the proportions of a compound in a mixture is um based upon the proportions of the elements in the mixture because right the elemental proportion is always fixed in a compound okay um so how can we express that so we can express how much an element is in a given compound by determining the mass percent of that element and the mass percent says that it is the um sorry the percent composition or mass percent of an element is the element's mass expressed as a a percentage of the total mass of one mole of that compound so we can see here if we have mass of the element divided by mass of the compound times 100 um and again this is in one mole so let's go ahead and look at an example so what is the mass percent of chlorine in uh chlorofluorochlorofluorocarbons or cfcs okay so what we did here is the first thing we established was the molar mass of the cfc right so we said okay there is um one carbon in here right carbon's mass is 12. um there are two chlorines in this compound chlorine's mass is 35.45 and then two fluorines with the mass of 19. so we add those all up and we know that one mole of chlorofluorocarbon has a mass of 120.91 grams so now we can say okay well what is chlorine in the cfc right so we know there's two chlorines in one cfc so we're going to take those masses and times 100. so chlorine's mass is the 2 times the 34. 35.45 right 70.9 out of the total 120.91 so this is the proportion that chlorine was in the total mass hence mass percent we multiply that by a hundred and we can see here that it makes about 58.6 percent of chlorophyll fluorocarbons and that is constant quite a tongue twister that is constant so we can use that as a conversion factor and say okay if i've got this much of this compound how much of this individual element do i have in the compound right so the mass percent um in of an element in a compound can be used as a conversion factor between the element and the compound so we have an example here where the u.s food and drug administration or the fda recommends that an adult consume about 2.4 grams of sodium per day now most of us get sodium from salt so if we know that we need to consume 2.4 grams of sodium per day how much salt would we need to consume well we need to know the mass percent of sodium in sodium chloride so sodium chloride is one sodium to one chlorine sodium has a mass of 22.99 chlorine has a mass of 35.45 so if we do that math beforehand we can see about 39 is chlorine or sorry is sodium in sodium chloride so then we can say okay if i've got 2.4 grams of sodium per day and i know that there's a hundred grams of i'm sorry there's 39 grams of sodium for every 100 grams of sodium chloride that's the mass percent i need to eat about 6.2 grams of sodium chloride a day we can then also use the mass percent to help us figure out how what is how pure is the sample and then what is a likely impurity in the sample again if we are looking at a compound we we know there should be a fixed ratio of the masses of the elements in the compound if we then look at the proportion of the elements in the sample we can say hey this sample has a higher mass percent of a certain element so maybe there's some impurity in there so again we can assess the purity of a um a pure sample a pure substance or the components of a mixture using mass percent so what we can say then is that if we have a higher mass percent of an element that's going to indicate that the impurity has a higher mass percent of that atom so that means if it is a compound the element will be combined with some sort of smaller um element okay so that just means that we're gonna have another smaller element in there okay um so let's look at an example so if we have a carbonate rock calcium carbonate so marble's a great example of calcium carbonate and we know there's enough another metal carbonate in there but we're not sure how do we know this well we know that in our sample we had 52 oxygen in that metal carbonate so we know there's some sort of other carbonate in there um so we're trying to figure out if it's magnesium carbonate in the sample or strontium carbonate in the sample along with the calcium carbonate so what we have to do is figure out what is the mass percent of oxygen in the magnesium carbonate and the strontium carbonate compare that to the mass percent of oxygen in the calcium carbonate and see which one explains the 52 percent oxygen in the sample so first thing we need to do is look at the mass of oxygens we know that in each sample there's three oxygens and there is a 48 grams in there okay so in calcium carbonate if we calculate the molar mass of calcium carbonate which i gave to you we had 109 grams sorry 100.9 grams per mole so 48 out of about 100 is 48 so oxygen in calcium carbonate is about 48 we can see here that we have a higher mass percent okay if we look at video here if we look at magnesium carbonate magnesium carbonate is about 48 or 80 48 out of 84 grams and that's about 60 and then strontium carbonate we see is 48 about out of about 148 which if we just rounded this to 50 and 150 that's about a third so we can see that in pure calcium carbonate we'd have 48 oxygen in magnesium carbonate we'd have 60 oxygen and then in strontium carbonate we would have 33 oxygen since our mass percent is higher than pure calcium carbonate which has to be in there well if you pick magnesium carbonate you'd be right right because the magnesium carbonate has a higher mass percent of the oxygen so therefore it must be the impurity in the sample so again we can use being able to calculate mass percent is a really great way to kind of figure out what is in our sample okay young scholars in this video we should learn how to we should have learned how to calculate the mass percent of an element in a compound so we take the mass of that element in one mole of the compound divide it by the mass of one mole of that compound right just like the moles of the element in the compound are a constant ratio the mass of that element in the compound should also be a constant ratio we can use that mass percent to convert between the mass of the compound and the mass of the element within the compound we can also use it to assess the components of the mixture or the purity of the mixture okay well that's all for today take care y'all bye