in this video we're going to be looking at how to calculate the number of moles if you are given the mass of a substance in grams or kilograms and if you are able to calculate the molar mass in chemistry we can convert between grams and moles using the following formula so we can convert between grams and moles by using something known as our molar mass the first Formula that we will go over when looking at how to calculate the number of moles have a look at this formula in more detail in is number of moles Baby M is mass and that is measured in grams and Big M is molar mass measured in grams per mole the unit makes sense because look at our other variables in our formula in being mole and Baby M being grams grams per mole and remember this formula allows us to convert between grams so mass and number of moles you can also represent this formula by using a triangle number of moles block it out is equal to mass Baby M divided by molar mass if I wanted to work out the baby M the mass in grams block it out that's equal to molar mass multiplied by n so this way is multiply if I want to work out the molar mass block it out it's equal to mass divided by number of moles so for example how many moles are there in 100 grams of NaOH if we take a look at our formula we can see that they are looking for moles they're looking for baby n how many moles they give me 100 grams that is mass and now you might be thinking but they didn't give me molar mass but as soon as they give you a compound or formula unit over here this is our molecular formula as soon as you're given that we can work out molar mass because remember in order to work out molar mass we use the periodic table and we use the atomic masses listed on the periodic table so we take our periodic table we look up the atomic mass for sodium for oxygen and for hydrogen we add them together and that gets us Big M so let's do it it often helps to list out the variables that helps us determine or decide which formula we want to use now just to show you my molar mass of 40 grams per mole I added up all the atomic mass numbers on the periodic table sodium's atomic mass 23 plus 16 which is Oxygen's atomic mass number plus hydrogen's atomic mass number which is number one gives me 40. now what you do is you write your Formula First you always need to write your formula first because in your exams you get a formula Mark then what we do is we substitute these values into the formula into the correct places so we're looking for number of moles then we substitute Mass which is a hundred grams into the formula that's Baby M you substitute 40 grams per mole in the place of Big M which is molar mass this will get you what we call a substitution Mark then you work that out you get your answer and you write it with your unit that'll get you your answer Mark if you leave out the unit you don't get that answer off in our second example they're also asking for number of moles so they're asking for n number of moles they give me the mass again 20 grams that is Baby M and they give me the compound aluminum sulfates now that doesn't give us the molar mass immediately but we know that we can work up the molar mass Big M by using the periodic table I'm going to list my variables write my formula and then I'll show you how we calculate it so what I've done is I've listed my variables I've calculated molar mass if you need help with how to calculate molar mass make sure to watch my video linked I've written down my formula now I'm going to substitute which means I'm going to put these values in the correct variables places so again you will get a mark for your formula you'll get a mark for substituting correctly and you'll answer Now take notes how when I do this on my calculator I get a decimal 0 comma zero five eight four the rule in physics is you need to round off in our case so in grade 10 physics how we do it we round off to at least two decimal places now you obviously need to read the question if they tell you a different number of decimal places or significant figures you need to follow that but in our tests we do two decimal places you do need to be aware of what the question is asking for because say for example I give you a question where I give you three more that is in so we know n is equal to three more I also tell you the compound so say for example it's calcium carbonate and therefore I give you the molar mass 100 grams per mole so that is Big M and I want you to work out Mass this is different because we're not calculating in but the steps that you follow are very similar so you list your variables you write your formula you substitute into your formula but in this case I have three more so that goes in the place of n I also know that I have 100 grams per mole that goes in the place of Big M so I'll leave baby M open because that's what I'm trying to find my mass so 100 grams per mole is Big M it's my molar mass that goes here and then you need to do slightly different order of operations in order to calculate your variable so in this case I'm looking for mass in order to calculate mass I simply say 3 times 100. remember we're solving for m m is my variable M baby Mass Baby M mass is being divided by a hundred so when we take that over we do the inverse Times by a hundred so mass is equal to 3 times 100 which is 300 grams just be careful of where your variables are and the Order of Operation that you need to do in order to solve for that variable another way that you could have done this is you could have looked at your triangle and you could have said well I need Mass masses in grams that's what I'm looking for in order to calculate mass I need to take my molar mass multiplied by my n remember I'm covering what I'm looking for and it's these two things multiplied by one another so molar mass 100 multiplied by number of moles three and there we go but remember it's very important you need to write your formula you need to show substitution you need to write your answer with a unit in the other videos in my playlist we look at how to calculate things like volume at STP and concentration of a solution using some of these other formulas remember to check out the links in my description below for more Stoichiometry videos more chemistry and more physics videos