Molarity vs. osmolarity

I want to talk about the difference between the two words-- molarity, M-O-L-A-R-I-T-Y, molarity-- and a word that's very similar, osmolarity. And I'm going to do it with a little example, because I think examples will help make this very clear. So I'm going to draw a box here. And any time I draw a box, just assume that's packed with one mole of some substance. And you know that 1 mole equals 6.02 times 10 to the 23rd. We know that's a huge number of little particles or atoms or whatever we decide to put in that box. So in this case, let's say, we have a few boxes. Let's say, we have one box here. And this box, I'm going to pack it full of this little green particle. And this is called urea. And if you're not sure what urea is, no worries, I'm going to draw it for you. And it's a molecule that our body makes to get rid of nitrogen. So you have little nitrogens here, two of them. And in between, you have a carbon and an oxygen, so sort of a small molecule. But it's a very useful molecule for helping us package up the nitrogen, so we can urinated it out or get rid of it some other way. So that's urea. Now, I'm going to draw two more boxes. And these boxes, I'm actually going to put something that you may be more familiar with, and that is salt. So I'm going to draw little sodiums, and next to them, little chlorides. So this is sodium chloride. And again, I'm just drawing a few of them. But just remember, because it's in a box I've got an entire mole of each of these things. So I've got here sodium chloride. And I'll try to keep the color code consistent. And I have two moles of it, so I've got an equal amount in either box. And now I've got three boxes of glucose. I'm going to draw glucose on this side. So you can see, I'm going from one box of urea, two boxes of sodium, to now three boxes of glucose. I'm going to just draw glucose as little red balls here. So each little red ball represents a glucose. And just to remind you what glucose looks like, we're going to draw it out as well. So glucose is a little molecule like this with an oxygen. And off of it, you get these little OH groups, so a little OH there option, oxygen and hyrdogen there. This one is like that. This one goes down. And you have another carbon coming off of it, with an OH as well. So that's your little glucose, and each little red dot represents one of those molecules. So we've got six moles of stuff here. And I'm going to make a little bit of space on this canvas. And we're going to say now, we're going to take our stuff and put it into a liter. So imagine I take a bucket or something here, and this is full of water, one liter of water exactly. So this is my little one liter. And you're going to take all this stuff, and let's say, dump it in here. So all six moles of stuff go in there. And now, I ask you, tell me the molarity of this stuff. So we have three things. So let's start with urea. What is the molarity of urea? Well, you'd say, well, I have one mole of it. And I have one liter, so one mole per liter equals one molarity. And a big M represents molarity. So that's easy to do. And then you have, let's say, sodium chloride. So you have NaCl. And you have two moles of it. We put in two moles of it into one liter. So you say, you have two molarity of sodium chloride. And finally, you have glucose. And you say, well, glucose-- and you're getting the pattern here. Three moles and becomes obviously same volume, and you have three molarity. So that's pretty straightforward, one, two, three. Now, imagine I actually take a little magnifying glass. I'm going to leave that up. Take a little bit of that water, and let's say, I zoom in on it. This is where things get really interesting. Let's say I zoom in on this a little bit of water right there, just to get a better look at what's going on. So I zoom in on it, and I get something like this. Let's see if I can draw it out for you. Oh, my circle is not so neat, but you get the idea. So you zoom in on that little circle, and here's what you might see. I'm going to draw the sodium first. So you might get something like this. Here's your sodium. And let's draw another sodium over here. And just to label it, so you know what it is. It's sodium. And it's positively charged. Don't forget. And sodium you positively charged, and we have some chlorides. And I'm not drawing them next to each other on purpose, because you'll see what happens. Even though sodium and chloride started out as partners. They started out next to each other. The moment they hit water an interesting thing happens. So the second they hit water, you've got H2O. And oxygen is slightly negatively charged. And let's draw oxygen there. And it's attached to two hydrogens, two little hydrogens like that. And this is your slightly negatively charged oxygen and your slightly positively charged hydrogens. And so that negative oxygen and that positive nitrogen attract each other. So it's going to line up like that. In fact, you might even get another oxygen over here, line up with its two hydrogens and maybe even another one over here. And you see what's happening is that, these oxygens and the hydrogens are lining up, so that the oxygens can be close to the nitrogen, or to the sodium, I said nitrogen by accident, sorry. And it happens over here too. Oxygen comes in close to the sodium, because it's got that little negatively-charged part to it-- call it a partial dipole-- and a little bit over here too. So some of that negativelyy-charged oxygen is being attracted to the very positive sodium. And actually, the opposite is happening over here. Here, you have these slightly positively-charged hydrogen, two of them. And those slight positive charges are attracted to the very negative chloride. So you have another one over there. And let's say, you've got some over here. So you get these little water molecules that are lining up next to sodium and chloride and basically getting between them, so they're not next to each other. So they basically start acting like their own little particles. Now, here's the key of osmolarity. Think about individual particles that are affecting the movement of water. And so really, sodium and chloride, they're not acting as one anymore. They're acting as their own individual particles. And you might be thinking, well, whatever happened to that glucose that was in the water. Did that disappear? And that's right there. Let's imagine little glucoses. And I'm drawing them very tiny, although we know that the molecule is actually pretty large. And here's our urea. So we haven't lost our urea and glucose. It's still there. But the key is that, they're lining up. The water is lining up so that it actually blocks out the sodium from the chloride, separating those two ions from one another, so that they behave as individual particles. So now, if you're looking at individual particles, how many individual different particles are there in this solution of water that's going to affect the movement of water? So we obviously have glucose. That's right here. And we have urea. That's right there. And now we have some sodium and four, we have chloride. So I'm really counting sodium and chloride as two separate things now, because they're separated out by the water. So now, if that's the case, let's go back to our question of molarity. And I'll write up here osmolarity now, osmolarity. And let's see if we can figure out the osmolarity of each of these things. So what is the osmolarity of urea? Well, for urea, we would say, well, there's still just that one mole in one liter. So that's going to be one osm. And we could say, well, I'm going to jump to glucose now. And sodium chloride, we'll do last. Glucose, we still have the three moles. And that's still in one liter. So that's three osms. And let me make a little bit of space here. And we have now sodium. And I'm going to do that as its own thing. And we have two moles. I should rewrite this. I've been writing moles, and that's not accurate. Now we're talking about osmoles. So I should write one osmole, three osmoles. You can see how similar the two concepts are. I replaced the words by accident. Here we have two osmoles of sodium in one liter. And that means that it's two osms. And finally, we have chloride. And that is also going to be two osmoles per liter. So really, when we started with sodium chloride and split up, we generate more osmoles, total osmoles. So if you're looking at total osmolarity, Total osmolarity here would be just adding it all up. So how many total osmoles do we have? We have one of urea, three of glucose, two sodiums, and two chlorides. We have eight osmoles. So if you wanted to calculate total osmolarity of this solution, you'd say, well, the answer is eight. And the simple way to that, of course, is just to say, well, we have urea, glucose-- and this is kind of the shortcut-- sodium chloride. And we have one here. We have three here. And we have two here, but you know it splits up. So you have to multiply by two, and then you just add it all up together. And you get eight. So that's kind of the quick way of doing it, but I wanted to show you the exact concept and what it would look like under a microscope, so that you understand exactly why it is that we end up having to multiply by two.