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Empirical Formulas

In chemistry, an empirical formula is a formula that describes the simplest whole-number ratios of atoms or molecules in a chemical compound. An empirical formula represents the simplest ratio of atoms of a particular element in a compound. For example, the empirical formula for glucose is CHO, because it is a simple ratio of one atom of carbon (C), two atoms of oxygen (O), and four atoms of hydrogen (H). The simplest ratio of atoms of a particular element in a compound is called its empirical formula.


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Video Transcript

empirical formulas are the smallest ratio off a molecular formula using integers. So, for example, if we have something like H 402 which doesn't exist, but for the purpose of this example, let's pretend that it does. The empirical formula would be H 20 because this is the smallest whole number ratio that you can write for this molecular formula. So while you're giving these types of problems, usually you will be given some kind of percentage composition. And from that you can calculate Thean parable formula of a molecule. So to illustrate this, I have a few examples. So let's say you're given that your molecule has an alum known percent composition of 52.9%. And let's say you're given an oxygen composition of 47.1%. So to calculate this, you can first, um, treat these percentages as cramps and then figure out the bulls and then the race ship Esos assert. Let's convert this into grams to make the calculation easier, so I'm simply rewriting this over. And then from grams. Let's calculate the number of most using the atomic masses of each atom. So for aluminum, um, will be using the fact that aluminum is 26.9 g per mole on DSO using dimensional analysis, you can get bulls, um, starting with the crimes. So to do this, you are going thio Multiply this like so So you want to make sure that the grams are on the bottom and moles are on top and you're going to set this up in the same way as aluminum and so far, oxygen you're going to use 16.0 And so when we do this, the grams cancel out and your answer is left and more so for aluminum, you're going to get one point to 96 moles and oxygen. You're going to get 2.943 moles and you can see that you're already getting some kind of number, so you can approximate this as to and oxygen as three. So from here we see that we can write the empirical formula for aluminum oxide, which is a little too 03 which makes sense because oxygen as an an ion has a minus two charge and aluminum has a plus three charge. So then, uh, this ends up checking out and This is your empirical formula. And so we'll be using this method to basically find the empirical formula Cave in the percent compositions eso for another example. Let's say you're given that your molecule has a nitrogen composition of 26 point 19% you're given the hydrogen is seven point five 38% and fluoride of 66 point to 7%. You're going to use the same method as before, so you're going to treat all these numbers as grams first. Now, let's write this over quickly, and then you're going to use dimensional analysis to calculate the number of moles. So, yeah, you're going to set this up in a similar way as before. Um, so for a nitrogen, you're going to use the fact that it is bomb 14. We're hydrogen. It's about one. So I'm using a shortcut here, but we're actually doing this calculation. Make sure you use the appropriate number of 66 so we're just all shock. Great. So I understand this to save a bit of time, but again, make sure that you actually use more number of significant than I am in this problem. And so if we calculates this. This ends up being 1.87 malls. This is seven point five three before walls and 1.87 Right. So in this problem, we can't really round this because 7.5, uh isn't really close to a whole number, like in the previous problem. So then what you want to do is you want Thio divide the whole ratio by the lowest mole. So in this case, 1.87 is most most. Then you want to divide this ratio, um, by 1.87 And so what you're left with is a ratio of one, 24 toe one with one being engine for being hydrogen and one being cl. So from here we can write the operable formula and age four CEO, which makes sense because this is ammonium chloride for ammonium chloride. Amazing neutral molecule, because Florida has a charge of minus one and the Poly Atomic Ion ammonium has a charge of plus one. So this makes sense. And so this would be your final answer. And so to illustrate this point again, we'll do one more problem and let me just erase this. So we have some more space to work with. Okay, so now for the next problem, let's say you have the following composition. So let's say you have 49 point. Um, sorry. Let's say you have 50 0.0% carbon and eight point 39 3% hydrogen and 66 0.61% auction. So again, we're going to convert this into Grams. Sorry, I realized that I did not label this Grams grams. Okay. And now we're going to multiply this. Sorry about that. Keep dropping these and then you're going to multiply this like before. So again used the appropriate number of favorite sick fix. But for the purpose of this video, we're going to take a shortcut to save time. And then you're going to multiply this like soap to make sure the grams cancel out. And then your lots with 4.16 moles for carbon, the point 3 to 6 holes for hydrogen and 4.163 for oxygen. And now you want to divide this? Why the lowest more value, which is 4.16 and then you should get a ratio of one, 2 to 1, which is C H to which is your empirical formula. So, um, we basically showed how you can derive the empirical formula for different molecules based on the percent composition given to you in the problem.