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Molarity vs. osmolarity

Molarity and osmolarity may sound similar, but they are two distinct concepts. Molarity (M) is the number of moles of solute per liter of solution. The unit of molarity is the mole (mol). Osmolarity (Osm/L) is the total concentration of all solutes in the solution. The unit of osmolarity is the osmol (osm). Osmolarity can be used to predict whether water will move from one side of a semipermeable membrane to the other. Created by Rishi Desai.

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  • blobby green style avatar for user Annika Johannsen
    I understand why NaCl spilt into Na and Cl, but why didn't Glucose split into O, C and H when put into water?
    (6 votes)
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    • leaf blue style avatar for user Esther Dickey
      The atoms are bonded differently. Na and Cl are held together by electric charges in an ionic bond. This kind of bond is relatively weak. Glucose is held together by covalent bonds, in which atoms share electrons. This type of bond does not break apart in water.
      (17 votes)
  • aqualine ultimate style avatar for user ❤MyHeadIsAJungle❤
    Does Osmolarity have to do with Osmosis? Sorry if i'm asking a stupid question, but i still have much to learn :(
    (5 votes)
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  • aqualine ultimate style avatar for user Diego Martinez
    At when he is writing it down, Is it important to make the OH upside down/reversed? I know its a low question but that's the only part i want to reassure myself with.
    (4 votes)
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    • piceratops seed style avatar for user Emily
      When he draws the ring structure of glucose, he draws some of the hydroxyl (OH constituent) groups pointing up from the ring and some pointing down because that's how they are in the actual molecule in three dimensions. The lines pointing up represent groups that are above the ring plane, while the ones pointing down show that those groups are below the ring plane.
      We know each carbon needs 4 bonds to be stable, so there must be a hydrogen there pointing the other direction, although we traditionally don't draw the hydrogens because it would be too cluttered.
      When the ring structure forms, the hydroxyls facing upwards are stuck facing up because there's a hydrogen in the way that keeps them from flipping to point down. Likewise, the OH groups pointing down are stuck pointing down.
      If one of those OH groups was flipped, the molecule wouldn't be glucose anymore; it would be another sugar like galactose or mannose.
      (2 votes)
  • blobby green style avatar for user laurenw258123
    What is the unit for osmolarity? Is it M?
    (4 votes)
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  • blobby green style avatar for user Gregory Borgman
    You mention ( around ) a carbon atom but I do not see It in your drawing. Did I miss something?
    (2 votes)
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  • leaf green style avatar for user Vijayraj Patel
    Why wouldn't glucose or urea split up?
    (2 votes)
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  • leaf green style avatar for user Mary Craven
    How does one calculate the molarity and then osmolarity of a 1.2% NaCl solution?
    (1 vote)
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    • leaf blue style avatar for user dysmnemonic
      The first step is to work out how much NaCl there actually is, by finding its mass and using n = m / M to get the number of moles. Divide this by the volume of solution (as c = n / v), and you get the molarity. It's easiest to just assume you have one litre of solution for both of these steps, because n mol / 1L = n mol/L.

      To find the osmolarity, you need the total concentration of solutes. Because NaCl dissociates into Na+ and Cl-, you'll have n mol of Na+ and n mol of Cl-. Add them together, and these will give you an osmolarity of 2n mol/L.
      (5 votes)
  • blobby green style avatar for user poojavasanthi
    Implies osmolarity applies only for ionic compounds.... right?
    (2 votes)
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    • female robot ada style avatar for user Lidiya
      No, osmolarity applies to any solute. It can be protein dissolved in water, or starch, or salts. Unlike salts, starch will not dissociate in water, therefore its osmolarity will be equal its molarity. However, the protein and starch osmolarity contribution to the total osmolarity of the solution will be negligibly small when compared to the osmolarity of salts dissolved at the same percentage in the solution because of the high molecular weight of the proteins and starch. For example, in a solution of 1 % starch and 1 % NaCl, when you calculate the total osmolarity, you can simply ignore the contribution of starch's osmolarity.
      (2 votes)
  • aqualine sapling style avatar for user Austin Guria
    How can I calculate the molality of a solution if I was given only the molecular wight of the solute (molecule or ion) and the solution density?
    (1 vote)
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    • leaf blue style avatar for user dysmnemonic
      An approach to this kind of problem is to look at what you need to find, and what things you already know.

      What you need to find for this solution is a molality - a concentration in moles of solute per kilogram of solvent.

      What you already know is the molar mass of the solute, so you know the mass in grams of a mole of solute. The other thing you have is the density, most likely as the mass in grams per millilitre (mL) of solution.

      If you know the density of your solvent, you can work out what volume is occupied by a kilogram of solvent. With that volume and the density, you can work out the total mass of the solution. From there it's possible to calculate the mass of solute, and then the molality of the solution.
      (3 votes)
  • blobby green style avatar for user 😊
    At it seems to me that he is making a mistake in calculating the molarity. When you add those solutes to 1 liter of solvent, the resulting solution will no longer be 1 liter, so the denominator fractions in the molarity calculation should not be exactly 1 liter. For example, 1 mole of glucose has a mass of 180g and the density of a 1 molar glucose solution is still close to 1 g/mL (1.066 g/mL). 1 liter of this solution has a mass of 1066 g but only 886 g of it is water, so it would have to be made up from about 890 mL of water. Another way to say this is that if you begin with 1 liter of water and add 1 mole of glucose, you will get 1180 g of solution whose volume will be 1.1 liters and therefore its molarity will be 0.9, not 1.0.
    (2 votes)
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Video transcript

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.