If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content
Current time:0:00Total duration:11:32

Video transcript

I want to talk about the difference between two words molarity mo lar ity molarity and a word that's very similar as molarity 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 then anytime I draw a box just assume that that's packed with one mole of some substance and you know that one 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 urea and if you're not sure what urea is no worries I'm going to draw it for you it's a molecule that our body makes to get rid of nitrogen so you have little nitrogen's 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 urinate 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 a little sodium's 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 a whole an entire mole of each of these things so I've got here sodium chloride 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 a glucose on side so you can see I'm going from one box of urea two boxes of sodium to now three boxes of glucose so I'm going to just draw a glucoses little red balls here so each little red ball represents a glucose and just to remind you a 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 H groups so little o H there oxygen hydrogen there this one is like that this one goes down and you have another carbon coming off of it with an O H 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 1 liter and you can take all this stuff and let's say dump it in here right so all six moles of stuff go in there and now I ask you tell me tell me the molarity the molarity of this stuff so we have three things right 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 tip 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 hat that pattern here three moles and becomes obviously same volume in your three molarity so it's pretty a four one two three right now imagine I actually take a little magnifying glass take a little bit of the I'm going to leave that up take a little bit of that water and let's say I zoom in on it okay this is where things get real interesting let's say i zoom in on this little bit of water right there just to get a better look at you know what's going on so i zoom in on it and I get something like this and see if I can draw it out for you so it's you know oh my circles 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 I'm going to draw the sodium first so my guess 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 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 kind of next to each other the moment they hit water an interesting thing happened so the moment the second they hit water you've got h2o right and you have oxygen is slightly negatively charged and let's draw oxygen there and it's attached to two hydrogen's two little hydrogen's like that and this is your slightly negatively charged oxygen and your slightly positively charged hydrogen's 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 it's two hydrogen's and maybe even another one over here and you see what's happening is that these oxygens and the hydrogen's 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 two 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 the negatively 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 right 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 get 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 that 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 chloride they're not acting as one anymore they're acting as their own individual particles right and you might be thinking well whatever happens that glucose I was in the water you know did that disappear and that's right there let's imagine little glucose is and I'm drawing them very tiny although we know that the molecule is actually pretty large and here's our urea right so we haven't lost our urea nitrogen urea and glucose it's still there but the key is that they're 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 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 right 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 all right up here osmolarity now osmolarity and let's see if we can figure out the similarity 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 right in one liter so that's going to be one awesome one awesome and we could say well I'm going to jump to glucose now and I'll go to sodium chloride will do last glucose we solve the three moles right three moles and that's still in one liter that's three awesomes 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 actually 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 Oz mole 3 oz moles you can see how similar the two concepts are right I replace the words by X and here we have 2 Oz moles of sodium in 1 liter and that means that it's 2 awesomes and finally we have chloride and that is also going to be 2 oz moles 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 total osmolarity here would be just adding it all up so how many total osmoles do we have we have 1 of urea 3 of glucose two sodium's and two chlorides we have 8 oz moles so if you want to calculate total osmolarity of this solution you would say well the answer is 8 and the simple way to do that of course is just to say well we have urea glucose this is kind of a 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 in our microscope so you understand exactly why it is that we end up having to multiply by two