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Current time:0:00Total duration:6:50

Empirical, molecular, and structural formulas

AP.Chem:
SPQ‑2 (EU)
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SPQ‑2.A (LO)
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SPQ‑2.A.3 (EK)

Video transcript

what I want to do with this video is think about the different ways to represent a molecule so the most obvious way is its name so for example you could be referring to a molecule of benzene benzene but just the word benzene tells you very little about what actually makes up this molecule and there's other naming conventions that do give more information but you might say well I actually want to know I want to know more about the actual particular elements that make it up well that might be in that case it might be useful to move up to the empirical formula empirical empirical empirical empirical formula and you might be thinking what does empirical mean well in general the word empirical is referring to something that's that comes from observation or comes through experiments if you could say hey you know I I from empirical evidence I now believe this this means that you saw data this means that you you you have some observations that make you think this new thing and the reason why we call what I'm about to write down the an empirical formula is because early chemists would has been able they can't look they weren't able to look at a just one molecule but they could at least come up with they could observe the ratios that it seemed like of the different elements that they had in a molecule so an empirical formula gives you a ratio of the elements in the molecule so an empirical formula for benzene is it is one carbon for every for every hydrogen now you might say okay that's a nice I now know that if I'm dealing with benzene I have one carbon for every hydrogen or one hydrogen for every carbon but what does but what what how many of each of these do you actually have in a benzene molecule and to answer that question that's when you would want to go to the molecular formula molecular molecular formula and the molecular formula for benzene which is now going to give us more information than the empirical formula tells us that each benzene molecule has six hydrogen's and a sorry six carbons and six I'm really having trouble today six hydrogen six carbons and and six hydrogens now the ratio is still one-to-one you get that right over here it's very easy to go from a molecular formula to an empirical formula you essentially are losing information you're just saying the ratio okay look it's a ratio of six to six which is the same thing as one to one if we wanted to we could write this as C 1 H 1 just like that to show us that the ratio for every carbon we have a hydrogen and we see that that's actually the case in one molecule for every six carbons you have six hydrogens which is still a one to one ratio but that might not satisfy you you might say well okay but how are these six carbons and six hydrogen's actually structured I want more information and for that you would want to go to a structural formula structural formula which will actually give you the structure or start to give you the structure of a benzene molecule a benzene molecule would be drawn like so you would have six carbons in a hexagon so one let me write it this way one two three four five six carbons in a hexagon just like that and then you have a double bond every other of these bonds on the hexagon is a double bond and then each of these carbons are also attached to a hydrogen also bonded to a hydrogen and each of these lines that I'm drawing this is a bond it's a covalent bond we go into much more depth than other videos on that but it's a sharing of electrons and that's what keeps these carbons near each other and what keeps the hydrogen's kind of tied to each or the hydrogen's tied to the carbons and the carbons tied to the hydrogen's so let me draw it just like this and this is only one variant of a structural it's hard for it's hard to see this that when I just drew so let me see if I can do a little bit that's about as good hopefully you see there's a hydrogen there and there's a hydrogen right over there this is one variant of a structural formulas strong structural formulas will actually give you some 3d information will tell you whether a molecule is kind of popping in or out of the page others might might not be as explicit in once you go into organic chemistry chains of carbons are just done they're just you might see something like this for benzene where the carbons are implicit as the vertex of each of or there's an implicit carbon at each of these vertices and then you say okay carbons got to have is typically not gana but it's typically going to have four bonds in its stable State well I only see one two three well if it's not drawn then there must be a hydrogen and that's actually the convention that people use in organic chemistry so there's multiple ways to do a structural formula but this is a very typical one right over here so as you see I'm just getting more and more and more information as to go from empirical to molecular to structural formula now I want to make clear that empirical formulas and molecular formulas aren't always different as if the ratios are actually the actual also show the actual number of each of those elements that you have in the molecule and a good example of that would be water so let me do water so let me do this in a different color that I I'm pretty much used every color so water so water we all know for every two hydrogen's for every two hydrogen's and since I already decided to use blue for hydrogen let me use blue again for hydrogen for every two hydrogen's you have an oxygen you have a high oxygen now it just so happens to be that this what I just wrote down I kind of thought in terms of empirical formula in terms of ratios but that's actually the case a molecule of hydrogen si a molecule of water has exactly two hydrogen's and and one oxygen and if you want to see its structural formula you're probably familiar with it or you might be familiar with it each of those oxygens that a hydrogen each of those oxygens in a water molecule are bonded to two hydrogen's are bonded to two hydrogen's so hopefully this at least it begins to appreciate different ways of referring to or representing a molecule
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