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

## Video transcript

I want to make it super clear what the difference is between a lot of these words that sound really similar but they're subtly different from each other so the first one is molarity and we know that that means moles in one liter of solution and 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 and that was the the major difference between the two of them right and so here you can see the denominator is just slightly different between these two words and if you now carry this on let's say we switch over to this side and we 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 so that's still the same but what changes is the numerator is now osmoles osmoles and you remember osmoles talks about not just what is in the solution but how things can split apart so we talked about salt as a great example of having a different molarity from osmolarity and rounding it out then you can imagine that there's a fourth word osmolality and osmolality is going to be the same numerator as osmolarity s can be osmoles except in the denominator we're going to pick up that one kilogram per of solvent definition so that's osmolality and 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 so that's that's four of the terms in the last term that we picked up was tone isset e Tona city so where does that fall into the mix Tona city and we even split it up into hypotonic solutions isotonic solutions and hypertonic solutions and we said that that's kind of how we think of tony city one of those three groups usually so 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 and these terms are used for two solutions so if you have two solutions separated by a membrane i should probably write that with a membrane then you can use these terms to describe how they are relative to one another so that's the key difference between these terms the first four are really for one solution and the tow necessity 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 so we usually use these in terms of medicine and in medicine you might have cells with a permeable membrane so actually let me make it a little permeable membrane and this cell is usually going to be sitting in some solution that solution could be the blood or it could be the interstitial fluid or some solution and when we talk about these four terms and especially tonicity we're talking about the relationship between this solution and the solution inside of cells so that's one way to frame all these different four words that we've talked about