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Molarity, molality, osmolarity, osmolality, and tonicity - what's the difference?

See how each of these terms tells us something different about a solution. Rishi is a pediatric infectious disease physician and works at Khan Academy. Created by Rishi Desai.

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Video transcript

Voiceover: I want to make it super clear, what the difference is between a lot of these words that sound really similar, but their subtly different from each other. So, the first one is molarity, and we know that means moles in one liter of solution. Keeping the numerator the same, but tweaking the denominator you get molality with an L. Now, molality is moles and here instead of a liter of solution we said it was one kilogram of solvent. That was the major difference between the two of them. Right? So, here you can see the denominator is just slightly different between these two words. If you now carry this on, let's say we switch over to this side, and we go to osmolarity. Osmolarity. We keep the same denominator. We don't change that, so, let me write that in. We say one liter of solution. That's still the same. What changes it the numerator is now osmoles. Osmoles. You remember osmoles talks about not just what is in the solution, but how things can split apart. We talked about salt as a great example of having a different molarity from osmolarity. Rounding it out, then you can imagine that there's a fourth word osmolality. Osmolality is going to be the same numerator as osmolarity, it's going to be osmoles. Except in the denominator, we're going to pick up that one kilogram per of solvent definition. So, that's osmolality. So, then, switching between these two, you can see the major difference here is just the numerator. Right? The numerator is different between these two. That's four of the terms, and the last term that we picked up was tonicity. Tonicity. Where does that fall into the mix? Tonicity. We've even split it up into hypotonic solutions, isontonic solutions, and hypertonic solutions. We said that's kind of how we think of tonicity, one of those three groups usually. Very broadly speaking, these terms, these four terms are really a way to define or describe one solution. If you have a solution you can describe it using these terms. These terms are used for two solutions. If you have two solutions separated by a membrane, as you probably write that with a membrane, then you can use these terms to describe how they are relative to one another. That's the key difference between these terms. The first four are really for one solution, and the tonicity terms describe how you might talk about two solutions separated by membrane, and keep in mind that all of this stuff, all of it, let me just make a little bit of space, is really trying to refer to medical terms. We usually use these in terms of medicine, and in medicine you might have cells with a permeable membrane. Actually, let me make it a little permeable membrane. This cell is usually going to be setting in some solution, and that solution could be the blood, or it could be the interstitial fluid, or some solution. When we talk about these four terms, and especially, tinicity, we're talking about the relationship between this solution, and this solution inside of cells. That's one way to frame all these different words that we've talked about.