this is an introduction to molecular formula and empirical formula we'll talk about what they are what the differences are between them and we'll learn how you write an empirical formula when you're given a molecular formula to start with here's a molecule of a compound ethene let's see how to write a molecular formula and an empirical formula for ethene now i often like to think of molecular formula as the regular formula which you may already be familiar with the molecular formula tells us how many atoms of each element are in a compound so ethene here has two elements carbon and hydrogen for carbon it has one two three four atoms so we do c4 and then for hydrogen we've got one two three four five six seven eight eight hydrogens c4h8 tells us how many atoms and of what type are in this compound now empirical formula over here is all about the ratio of different types of atoms in a compound okay so here we have four carbons and eight hydrogens let's write this as a fraction or as a ratio okay so we got four carbons four carbon over eight hydrogen now what's special about the empirical formula is it tells us the simplest or most reduced ratio of the atoms in a compound so if this ratio or fraction here were on your math homework how would you write this in the simplest or most reduced form okay you'd think what was the largest number that you could divide both of these by and in this case it's four we can divide the top by four and we can divide the bottom by four and when we do that we'll get four divided by four will give us one carbon over eight divided by four which is two hydrogens and this is now the simplest or most reduced form of this fraction ratio now we write the empirical formula based on this simplified ratio okay so it's going to have one carbon c we don't write anything after it because it's just one and then h two hydrogens h2 so ch2 is the empirical formula that represents the simplest or most reduced ratio of the atoms in the compound whereas a molecular formula tells us how many atoms of each element are in the compound let's look at another here's a molecule of the compound cyanotriazide i think this molecule has a really cool shape it looks like something out of star trek or something okay so for the molecular form how many atoms of each element do we have here in this compound okay so we got carbon and we have one two three of them so c3 and then we got nitrogen and we got one two three four five six seven eight nine 10 11 12 n 12. that is our molecular formula c3 and 12. now for the empirical formula we want to take this first and write it as a ratio okay so three carbons over 12 nitrogen and we want to ask ourselves how can we simplify this as much as possible what's the biggest number that we can divide both of these by in this case the largest number is three we can divide both the top and the bottom by three and that will give us one carbon over four nitrogen is the most simplified ratio of elements for this compound and now we write the empirical formula using this most simplified ratio so we'll have c 1 so we don't put anything after it and then we have n 4 so cn4 is the empirical formula here now not every compound or molecule has only two elements here so we can't always get an empirical formula by just simplifying a fraction but even when we have a molecular compound that has more than two elements the basic math steps we use are still totally the same check out this molecular formula that has carbon hydrogen and oxygen in it we want to write an empirical formula for it we can't really write this as a fraction like we did with the previous ones but we'll look at these numbers 5 10 and 5 and ask what's the largest number that we can divide all three of them by for c5 h1005 that number will be five we want to divide everything by five so for the empirical formula we'll get c 5 divided by 5 gives us 1 so we don't put anything after it h 10 divided by 5 gives us 2 so we'll do h 2 and then o 5 divided by 5 gives us 1 again so we don't put anything after it here so this is the empirical formula for this molecular formula we just want to ask what's the largest number that we can divide each of these subscripts by in the molecular formula to get the empirical formula now with some molecular formulas you just can't simplify them anymore for example p3 and 5 okay there's no number that we can divide both of these by to simplify it further the same is true for c5h12 there's nothing we can divide both 5 and 12 by when this happens we keep this in mind that if the ratio of atoms in the molecular formula can't be simplified anymore the empirical formula is the same as the molecular formula so for p3n5 its empirical formula is p3n5 and for c5h12 its empirical formula is just c5h12 don't get confused by this teachers and textbooks love to give you molecular formulas that you can't reduce anymore and then ask you what the empirical formula is this freaks a lot of students out don't be freaked out by it just remember that if you can't simplify the molecular formula anymore there's nothing wrong it just means that the empirical formula is going to be exactly the same as a molecular formula now finally i want to mention that many different compounds can all have the same empirical formula early in the video we saw the molecular formula c4h8 has ch2 as its empirical formula but many other compounds also have ch2 as their empirical formula c2h4 c3h6 c5h10 and c6h2 all have ch2 as their empirical formula the empirical formula as we've said is all about the most simplified ratio of atoms in a compound so whenever we have a molecular formula with just carbon and hydrogen where we have twice as many hydrogens as carbons c2h4 c3h6 the empirical formula will always be ch2 so many many molecular formulas can have the same empirical formula keep that in mind so now you've learned about the difference between molecular formula and empirical formula the molecular formula tells us the total number of atoms of each element that are in a compound and the empirical formula is the simplest or most reduced ratio of those atoms we saw that some molecular formulas just can't be simplified anymore so the empirical formulas are the same as the molecular formulas and finally we saw that many compounds with different molecular formulas can all have the same empirical formula so now that you've learned this you might want to go on to the writing empirical formula practice problems or you might want to watch my video called what's the point of empirical formula