Empirical Formula and Molecular Formula Introduction

This is an

introduction

to the

molecular

formula

and the

empirical

formula

, we will talk about what they are, what the differences are between them and we will learn how to write an

empirical

formula when you are given a

molecular

formula to start here you have a molecule of an ethene compound let's see how to write a molecular formula and an empirical formula for ethene now i often like to think of the molecular formula as the regular formula that you may already be familiar with the molecular formula tells us how many atoms of each element there are in a compound so ethene here It has two elements carbon and hydrogen for carbon it has one two three four atoms so we do c4 and then for hydrogen we have one two three four five six seven eight eight hydrogens c4h8 tells us how many atoms and what type are there in this compound?

Now the empirical formula here has to do with the ratio of different types of atoms in a compound. Well, here we have four carbons and eight hydrogens. Let's write this as a fraction or as a proportion. Well, we have four carbons. four carbons over eight hydrogens, what is special about the empirical formula is that it tells us the simplest or smallest proportion of the atoms in a compound, so if this proportion or fraction here were in your math homework, how would you write this in the simplest way or more? reduced form, well, you would think what is the largest number that you can divide both by and in this case it is four, we can divide the top part by four and we can divide the bottom part by four and when we do that we will get four divided by four we will give one carbon over eight divided by four, which is two hydrogens and this is now the simplest or most reduced form of this fraction relationship.

Now we write the empirical formula based on this simplified relationship, okay, so it will have one. carbon c we don't write anything after because it's just one and then h two hydrogens h2, so ch2 is the empirical formula that represents the simplest or smallest proportion of the atoms in the compound, while a molecular formula tells us how many atoms of each element is in the compound, let's look at another here is 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, so for the molecular shape, how many atoms of each element do we have? we have here in this compound, okay, so we have carbon and we have one two three of them, so c3 and then we have nitrogen and we have one two three four five six seven eight nine 10 11 12 n 12. that's our molecular formula c3 and 12.

Now for the empirical formula, we want to take this first and write it as a proportion. Okay, so three carbons over 12 nitrogen and we want to ask how we can simplify this as much as possible. What is the largest number that we can both divide? of these 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 relationship is the most simplified, so we will have c 1, so we don't put anything after it and then we have n 4, so cn4 is the empirical formula here.

Not all compounds or molecules have only two elements here, so we can't always. obtain an empirical formula by simply simplifying a fraction, but even when we have a molecular compound that has more than two elements, the basic mathematical steps we use remain totally the same. Check out this molecular formula containing carbon, hydrogen, and oxygen. write an empirical formula, we can't really write this as a fraction like we did with the previous ones, but we will look at these numbers 5, 10 and 5 and ask what is the largest number we can divide all three by. for c5 h1005 that number will be five, we want to divide everything by five, so for the empirical formula we will get c 5 divided by 5, which gives us 1, so we don't put anything after h 10 divided by 5, which gives us gives 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 here, so this is the empirical formula for this molecular formula, we just want to ask what is the largest number that we can divide. each of these subscripts 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 by to simplify it further. is true for c5h12 there is nothing we can divide 5 and 12 by when this happens, let's keep this in mind that if the ratio of atoms in the molecular formula can no longer be simplified, 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 simply c5h12 don't get confused with this, teachers and textbooks love to give you molecular formulas that you can no longer reduce and then ask you what the empirical formula. many students don't be scared by this just remember that if you can no longer simplify the molecular formula there is nothing wrong it just means that the empirical formula will be exactly the same as a molecular formula now finally I want to mention that many different compounds can have the same formula empirical formula at the beginning of the video we saw that the molecular formula c4h8 has ch2 as an empirical formula, but many other compounds also have ch2 as an empirical formula c2h4 c3h6 c5h10 and c6h2 all have ch2 as an empirical formula the empirical formula as we have said has to do with the proportion most simplified number of atoms in a compound, so whenever we have a molecular formula with only carbon and hydrogen where we have twice as many hydrogens as carbons c2h4 c3h6 the empirical formula will always be ch2 as many molecular formulas can have the same empirical formula, have it consider.

Now that you have learned 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 smallest proportion of those atoms. We saw that some molecular formulas simply cannot 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 have learned this you might want to move on to writing empirical formula practice problems or you might want to watch my video called What's the Point of the Empirical Formula?