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let's talk about the difference between two words molarity and molality so the first one has obviously a little R in it and the other has an L in it and in fact sometimes when people say it quickly it's hard to even hear which one they just said so just make sure you listen carefully because there is a slight difference actually two little differences that we'll talk about so molarity let's start there is really talking about moles of something some particle over one liter of solution and I haven't been writing out liter of solution but that is what is meant so one liter of solution that is molarity now molality is slightly different so let me do in a different color molality is actually the moles again so that part is the same over one kilogram so this is now actually looking at mass one kilogram of solvent so not solution but solvent and so let me make that very clear so this is one liter of solution this is one kilogram of solvent so both are looking at the same numerator but the denominator is different so let me actually draw out an example of what this might look like let's say we have our solution down here and let's say it's mostly water so I'm going to fill this in with water and let's fill it in more quickly using a little airbrush so let's say this is our solution of water I'm going to make it nice and even you can see the line that it goes up to so this is carefully measured out carefully measure it out and I'm going to put one mole of something we can decide whatever that something is going to be in in this case I don't know let's say we decide to put some urea in there little molecules of urea and we know that urea is something our body uses to get rid of nitrogen so something that we throw into urine and actually even sounds like urine so I'm going to put some molecules of urea in here and we're going to make our urea I don't know let's say kind of a pink color this is our urea and this is one mole of it so I'm only going to draw a few of the little molecules but you know that whenever I say there's a mole that means mean that there's 6.02 times 10 to the 23rd of these molecules in here so lots and lots of molecules so one mole this refers to 6.02 times 10 to the 23rd so lots of molecules hanging out in our solution now and so let me actually then now cut and paste this we're going to cut and paste this over to the other side so it's over here and we'll move this underneath here so you know this is exactly the same right so this is just the exact same solution and I still have my little pink urea and now on this side let's say I measure this out and check the level here let's say this level right here is that is exactly one liter well then I would say this solution has one mole of urea in one liter of solution so we have one molar solution of urea so that's our molarity right so so far so good and these are little ureas just to make sure that we're clear about that so this is our molarity but what about the other side our molality so for that I actually need to use a little eraser so imagine now that I actually removed all the urea because I don't want the solution I just want the solvent I just want the part that is water I don't care about the molecules I just want to first way get a one kilogram of the solvent so to do that I've got to get rid of all my urea right so I take out all the little molecules of urea and immediately or you can't mention that the water is actually going to allow those little holes to be like that right they're going to fill in those holes immediately so right away those holes are going to get filled in right so let's fill them in with water so the water rushes into those holes and fills them in but in doing so in filling in these little holes of course the level falls right you actually have a little bit less less water so you actually drop the level of water a little bit let me erase some water up here because the water level falls just a little bit to fill in all those holes of solvent that I took away or all the holes of urea that I pulled out so now my level has fallen and so if I was to measure this let's say this is a less than 1 liter let's say it's 0.99 liters it's going to be very close but it's going to be slightly less right so let's say about 0.99 liters so it's a little bit less than a liter and remember one liter equals one kilogram so for water and most temperatures one liter for water one liter equals one kilogram so I guess I have to ask the question does this equal 1 kilogram well the answer is no right it actually equals about zero point nine nine kilograms it's actually slightly less than a liter so that's going to weigh less than one kilogram just point nine nine kilograms so now I have really one mole of urea thinking back to how much I dumped in I had put in one mole of urea I'm just going to let it hover here because this is where it was right before it fell into my water one mole of urea was going in to only 0.99 kilograms of water and so if that's the case then my molality is actually going to be slightly different it's going to be one mole of urea of urea over point 9 9 kilograms 0.99 kilograms of solvent of water and so one divided by a number slightly less than 1 will be a little bit more than 1 so my molality will actually be maybe let's say one point zero one or thereabout it'll be just slightly upwards of one and that will be the molality molality so they're very similar right like one molarity in this case was going to equal just a little bit higher molality and that's because we know that the the molecules of urea take up a little bit of volume and that makes the overall volume of the solvent a little bit less so that's the key difference and if you think about this really when you're talking about blood and things that are dissolved into blood most clinicians will jump back to molarity because just easier to work with and you don't have to actually figure out the exact amount of solvent you can just think about the solution so most of the clinicians or doctors and nurses will think in terms of molarity but most of the time when you're working in a lab setting and you can be more precise people think about molality