percentage composition is a very common thing that you'll be asked in the quantitative aspects of chemical change or stochiometry you'll be asked this in grade 10 11 and 12 you need to know how to do this sometimes the question isn't very obvious sometimes they don't say work up the percentage composition they might say work out the percentage by mass we will deal with a lot of questions like that in this video Let's Jump Right In before we speak about percentage composition in terms of the quantitative aspects of chemical change chemical reactions or stochiometry I just want to explain in Easy terms what does it mean if I ask you for the percentage composition of something so if I give you a bag okay here's a bag and I say 30 G of the bag is rice 40 G is beans so what I mean is the stuff filling up the bag 30 G of what is filling up the bag is rice 40 G of what is filling up the bag is beans what is the percentage composition of the substances that is filling up the bag bag how would you work that out well you know that in total the substances that are filling up the bag is going to be 30 + 40 G so the total amount of substances filling up the bag is 70 G so if I want to work out what is the percentage of the bag that is filled with rice how do I do that well I know that Rice the amount of rice in the bag is 50 g the total mass of the bag is 70 G and I times it by 100 in other words at the bottom of your fraction is the total mass or the total amount the total mass of the bag total mass of the content of the bag and at the top is the mass of the substance that we care about so because I'm working out the percentage of the bag that is rice at the top I have the mass of rice or the mass of the specific substance and why do I times it by 100 you times it by 100 because we're working out a percentage so if you work that out you get 42.86% I rounded it off to two decimals with a percentage sign that is the percentage of the bag that is rice obviously to get the percentage of the bag that is beans you could say 100% minus 42.86% to get the percentage that is beans or what you can do is the following the top of the fraction we put the mass of the beans which is 40 g at the bottom we put the total mass of the bag times 100 and we get 571 14% when we work out the percentage composition we are working out in this case percentage composition by mass so the bottom of the fraction is always going to be the total mass the top is the mass of the compound or the element or the substance that I care about so when it comes to working out percentage composition or percentage by mass of a specific compound so say for example the first example that we do is H2O if I want to work out the percentage composition of H2O we know that there are two elements that make up H2O two elements that make up this compound we have hydrogen which is the H and we have oxygen which which is the O oxygen so basically we are going to be working out the percentage that is hydrogen and we're going to be working out the percentage that is oxygen what percentage of water is hydrogen what percentage of water is oxygen and to do that we need two things at the bottom of the fraction it's always going to be the total mass of the compound in other words the total mass of water and when I say total mass we're going to be working with molar mass which means we need to work with the atomic mass numbers which comes from the periodic table there we go that's the denominator of the fraction and the numerator of the fraction is the mass of the elements in that compound so these would be our steps work out the total molar mass of the compound as I said we need to use the atomic mass numbers we need to use the periodic table and we're doing it for the total compound then step two is work up the mass of each element within the compound so step one will give us my denominator and step two will give me the top of the fractions the numerators we use the above formula which is this formula in order to work out the percentage composition if the question says give me the percentage composition of H2O of water we do it as follows let's do step one work at the total molar mass of the compound when working at the molar mass of H2O we need to look at our periodic table hydrogen there it is the atomic mass number atomic mass number of hydrogen is 1 but remember it's H2O we have two hydrogens so it is 1 * two plus now we need to look for oxygen oxygen is over here the atomic mass number for oxygen is 16 there's only one oxygen so we say + 16 our total molar mass therefore is 8 18 G per mole now remember this is the total mass of water this is the denominator of my fraction step two work out the mass of each element within that compound so essentially what I'm trying to do is I want to work out what percentage of water is hydrogen and what percentage of water is oxygen there's only two elements in H2O the denominator remember is the total mass 18 18 we're going to times both fractions by 100 because it's percentage composition now remember this is step two so what is at the top of this fraction the top of this fraction is the hydrogen within H2O so it's this part of my calculation hydrogen has a mass of one but there's two of them in water H2O so it's 1 * 2 so basically it's 2 over 18 * 100 and I'll work that out now then for oxygen oxygen is 16 so it's 16 over 18 * 100 there's only one oxygen so 16 * 1 essentially so 16 over 18 * 100 and the answers are as follows I have rounded both of them off to two decimal places so what this means is that 11.11% of water consists of hydr 88.89% of water consists of oxygen let's do another example slightly more difficult because the chemical compound looks a little bit interesting so it says give the percentage composition of magnesium hydroxide now magnesium hydroxide first thing that we need to consider is the fact the question says give the percentage composition what that means is you need to consider what elements exist in this compound we have magnesium we have oxygen and we have hydrogen so because the question wasn't specific the question didn't say what percentage of the compound is consists of oxygen no they said give the percentage composition so the breakdown of the compound they want to know what percentage of the compound is magnesium and what percentage is oxygen and what is hydrogen so breakdown telling me the percentage of each element in that compound Okay cool so step one work out the molar mass of the entire compound so Step One is we need to work out the molar mass of mg2 let's look at our periodic table so the first element that I care about is magnesium and the mass atomic mass number for magnesium is 24 and if you look at my compound there's only one magnesium okay so just 24 plus my next element so we did magnesium tick my next element is oxygen now very very important and some students don't understand this because there's brackets around the O and there's a little two outside the bracket think about maths it's like the distributive law so you multiply the two into the bracket so the two belongs to the oxygen and the two belongs belongs to the hydrogen what that means is that there are two oxygens two oxygens and two hydrogens so if we look at our periodic table oxygen is 16 but there's two of them so 16 * 2 plus hydrogen periodic table hydrogen is one but there's two of them so 1 * 2 if I add these together 24 + 16 * 2 + 1 * 2 you get the molar mass for this entire compound which becomes the denominator of my fractions I get 58 G per mole that is the denominator of my fractions so when I'm working out the percentage of magnesium 58 goes at the bottom okay it's just times 100 when I'm working out the percentage of oxygen 58 goes at the bottom it's the to total when I'm working at the percentage of hydrogen 58 goes at the bottom I think you you are all understanding what I mean by now then the top of the fraction is just the Magnesium so if you go back to what we just calculated over here this 24 is just the Magnesium okay the oxygen is this 16 * 2 it has to be 16 * 2 it can't just be 16 because as we said there's two oxygens in this compound hydrogen is 1 * 2 because there's two hydrogens in this compound then we can type that into our calculator and we get the following answer there we go there's my three percentages and take notes if you add these three percent percentages together it should give you 100 it might not give you 100 exactly because we rounded these off to two decimal places my last question which is a little bit more difficult so for those of you that want a challenge here it is see if you can try it yourself so pause the screen see if you can do it but this question says calculate the mass of nitrogen in a 30 kilog bag of ammonium nitrate nh4 NO3 so this is my substance okay I have a 30 kilog bag of nh4 NO3 ammonium nitrate I want to know of this 30 kg what is the mass of nitrogen in that bag so is the nitrogen 2 kg is it 3 Kg is it 4 kg what a lot of people would do which is wrong is a lot of people would say well there's one two nitrogens so wouldn't it be 2 kilog or some people would say well there's two nitrogens and the total bag is 50 kg that doesn't make sense okay you can't there's two is the number of nitrogen atoms in this compound 30 is kilograms of the bag so what you're going to have to do first if you haven't paused it yet try and pause it see if you can figure it out what you first need to do in this question is you first need to figure out what percentage of this compound what is the percentage of nitrogen in nh4 NO3 so does nitrogen make up 50% of the compound does nitrogen make up 25% of the compound does it make up 100% definitely not of the compound obviously nitrogen cannot make up 100% of this compound because there's hydrogen there's oxygen but if we know the percentage of the compound that is nitrogen we will then be able to work out the mass of nitrogen I hope that makes sense so let's pretend it's not but let's pretend that we worked out that 50% of this compound is nitrogen so 50% is nitrogen that means that 50% half of the compound is nitrogen it means that half of this mass will be nitrogen which means that nitrogen would be 15 kilg does that make sense I hope it does so step one is to work out what is the percentage of nitrogen in this compound and how do you do that how how do you work out the percentage of nitrogen in a compound it is percentage by mass so you do it just like we've been doing the other examples in the question step one is to work out the molar mass of the entire compound so let's do that together so the molar mass of nh4 NO3 what elements do we have in this compound we have nitrogen there's actually two nitrogens we have hydrogen and oxygen so these two nitrogens and what is the mass of each nitrogen here's nitrogen 14 so it is 2 * 14 plus we have four hydrogens and you should know now that hydrogen is 1 * 4 remember there hydrogen is 1 but there's four of them so 1 * 4 and oxygen is 16 * 3 just to show you here's Oxygen 16 * 3 you work that out and you should get 8 G per mole so that is the total mass of the compound now remember the question wants me to calculate the mass of nitrogen so I want to work out what is the percentage of nitrogen so the percentage of nitrogen is equal to remember the top of the fraction will be the mass of nitrogen the bottom is the total so the bottom is the total that we just worked out the top is the mass of nitrogen here 2 * 14 remember we said that there's not one so my compound is nh4 N3 there's not just one nitrogen there's two nitrogens so two of them and the mass of nitrogen is 14 divided by 80 because 80 is the total mass of the compound time 100 and you should get 35% now remember in previous percentage composition questions like this one that we did before and like the one with water we worked out the percentage of all the elements we did magnesium and oxygen and hydrogen in this one I'm just doing nitrogen I'm not doing the hydrogen I'm not doing the oxygen the reason why I'm not doing that is because they didn't say calculate the percentage composition of nh4 NO3 they didn't ask for all the elements they want the mass of nitrogen I only care about the percentage of nitrogen so I just want to put into context what this means so far it means that I have this bag okay this 30 kg bag let's forget about the mass of the bag for now I have a bag and I'm telling you that 35% of that bag is nitrogen 35% of that bag so if I want to know the mass of nitrogen I know 35% of the bag is nitrogen so therefore the mass of nitrogen will be the 35% of the 30 kg okay 35% of and of su is a multiplication sum 35% percent means we divide by 100 35% of the 30 kg bag is nitrogen that the nitrogen is [Music] 105 kilog sorry kilog of the bag I really hope that makes sense this is one of the more difficult ways that they can ask percentage composition I hope that that's been helpful if you'd like more examples like this of the qualit quantitative aspects of chemical change or stereometry some of these examples come from my study guide which you can purchase on my website it contains 50 worked examples it covers this topic from grade 10 through to grade 12 it is a difficult topic so if you struggle I would recommend this it's 100 pages long I hope to see you in another video very very soon remember to watch the rest of this playlist for the rest of this topic bye everyone