 The test was the first experiment and it did not give the desired outcome. Many people are currently getting an F, which means that you got below 40. That's not fatal, but it's not good. The highest score was 105, but it was possible to do well. It wasn't an impossible exam. But I think quite honestly it must have caught some of you by surprise. What you have to ask yourself is, what was the hang-up? Did you not know what you were doing? Did you know what you were doing but you couldn't execute it? Were you careless? Did you get mentally flustered? All those can be issues. Performance anxiety can wreck your frame of mind. If that's your problem, rehearse. Take the exam. Sit at your desk at home. Ten minutes on this problem. Ready? Go! And think, if I don't get this right, I'm going to get an F. Become an actor or actress. Get the adrenaline flowing. And then practice performing under those conditions. That's the way to improve if performance anxiety is the problem. If it's lack of understanding, that means you have to study harder. That's the only cure for that. Learning is a chemical reaction. You have to drive it with your force of will or it doesn't happen. You don't get stronger for no reason or by lifting a feather. Or in fact by doing anything you can do easily. And you don't get smarter or more proficient by just looking at the answer and never actually working it out. It's easy when you see somebody else do it to think, yeah, I could do that if I wanted to. But, you know, I got a lot of things to do today and that's not one of them. So I'll do that tomorrow. That's a problem for some people. You're paying a ton of money to come here. At least from my perspective, it's a ton of money. And you have to make sure that you get every dollar's worth. I'm trying to give you good value, but I'm only half of the equation. The other half is how hard you work. Some of you worked extremely hard, but honestly if you got a ten, you really need to look at what you're doing. And you need to change it quickly because you won't pass the course with that kind of performance. You get five for the exam form. Alright, enough said. I'm going to pick out the problems that I was particularly worried that the percentage of wrong answers was high and those problems are going to make an encore appearance on another exam. So study this exam and make sure that you can do all these problems. That's the best way to do it. Okay, let's continue on with chapter 12. Solutions and Collegative Properties. Collegative properties are properties that depend on the number of particles in solution, but not on the details of the kinds of particles, at least not so much. There are, first of all, let's talk about concentrations. There are three common ways to express concentration of a solute and a solvent. Theoreticians like mole fractions because theoreticians don't have to actually measure anything and so they don't have to actually make the solutions up. But they have very nice theories where the natural concentration is the mole fraction. That's just the natural thing to use. It drops out of the equations. It makes everything so simple they love it. But an experimentalist doesn't like mole fractions because then you have to work out the moles of everything and it can be tedious. But the mole fraction is just the number of moles of whatever you've got divided by the total number of moles. And it's given this symbol, chi, fancy looking x. I may have told you we use Greek letters to keep out the riffraff in science. Greek letters intimidate people and then you find out who's not easily intimidated. So for example, if we take a tenth of a mole of sodium chloride and we have one mole of water, then the mole fraction of sodium chloride is just 0.1 over 1 plus 0.1 or 0.09. As I say, theoreticians like it but it's not much used in the lab. Chemists want to make up solutions quickly and they want to make them up reproducibly. And so the fewer things they have to measure or fiddle around with, the better it is. You have to weigh out, usually you have to weigh out what you're going to dissolve in the solution or if it's a gas you have to know the pressure of gas you're putting in. But that's not such a big deal because those things are usually easily measured and solids don't slosh around and it's not too bad. But when it comes to the liquid there's an easy dodge and that is you engineer the amount of liquid after the fact. And the most common measurement then is molarity with an R and that's the moles of solute per liter of solution. Again, chemists like to know how many moles are in there because that has to do with how many atoms are in there and that has to do with how many little chemical reactions can occur. If I just know how many grams of something are in there I don't really have a clear idea of what's going on. So we weigh out the solute and then we just dump it in and then we make up the solution to a mark on a calibrated volumetric flask at the right temperature which is usually 25 Celsius and we're done and that's easy to do because you don't have to weigh out the water or put in a certain number especially if you have to put in a certain amount of volume of water it's a nightmare because the water sticks to the container that you use to pour it in and there are some left behind so that's a big problem. So to get rid of that we define molarity to be moles of solute per liter of solution and we don't want to have to measure the volume of the solvent and the glassware companies will sell you any size you like because they're common things to find in the lab and these are probably just food coloring who knows but in any case you throw in the solid and you make it up to the mark first you add distilled water quickly and swirl it around and that's why it has that shape so you can mix it easily not drop it on the floor and then it narrows here so that you can get good accuracy on the scored mark you don't want a big wide thing there a little height is a lot of volume yeah yeah yes if you try to make too concentrated a solution let's say you don't know and you try to make 10 molar of something so you pour in 10 moles of this stuff and then you throw in the water and it doesn't all dissolve then you've got to start over and if you pour in a liquid and another liquid and they form two layers that's it that you aren't making a solution you're just making two layers like salad dressing ethanol would be a special case because ethanol is volatile so when I pour in the ethanol I have to worry about some of it actually evaporating if I go away and that type of thing so I have to be a little bit more careful with that if I'm using a volatile thing but ethanol and water will mix in all proportions but as I'll show you they don't make an ideal solution they make a solution but it's not what's called an ideal solution that's a special term like ideal gas this is the most common thing you're going to run into and it's the easiest to use no question about it and then courtesy of some very important experiments it turns out the final case instead of big M for molarity is little M for molality and again I told you you don't want to have to measure the volume of solvent but you don't mind weighing the solvent because I can sit there with the volumetric flask and I can pour in solvent while it's on a balance and if I pair the balance I can add a kilogram of solvent no problem again that's pretty easy to do because I can add drops at the end and watch the balance and this turns out to be important in certain kinds of experiments that I'll talk about and so that's also a very useful thing to be able to do in an analytical chemistry lab and the key is we weigh the solvent but we don't have to measure its volume these colligative properties there are a bunch of them boiling point elevation freezing point depression osmotic pressure and so forth they all depend on the molality that's what drops out of the theory that you'll do in physical chemistry let's practice converting concentrations these numbers are not exact but let's let's just use them ok they are I don't think they're quite right because the molality comes out too high as to what I recall but let's try it let's assume that sulfuric acid is 18.4 molar and has a density of 1.82 grams per cubic centimeter what is its molality ok I need to think what I'm going to do first of all I know if I have a leader of solution I know how many grams that weighs because I know how many grams a cubic centimeter weighs and I know how many moles of sulfuric acid are in there so if I look up the molar mass of sulfuric acid then I can tell how many grams of sulfuric acid is there the rest of it is water because while volumes do not add masses do that's why we like to stick to calculating masses if you mix two liquids together or you put a solid in a liquid the volume won't add up in fact it can sometimes actually get a little smaller even though I add something because something I add can break up the structure that's kind of pushing the thing apart and actually make it shrink and that's very confusing then to try to measure things by adding up volume so we never ever do that in chemistry ok let's have a look there are 18.4 moles of sulfuric acid per liter of solution those 18.4 moles will have a mass of 1800 and 4.7 grams based on the density of 1.82 and here I'd like to have some more digits let's take 1.82 a thousand cubic centimeters or one liter of solution would have a mass of 18.120 grams out of the 18.20 almost all of it is sulfuric acid there's hardly any water so this is really really concentrated solution we can do that because sulfuric acid itself is a liquid it's a very very viscous liquid itself it's not going to solidify on us and come out of solution well out of these 18.20 1804.7 are H2SO4 molecules and that means the other 15.3 are water and now for molality I have to figure out how many moles I would have in a kilogram of solvent but a kilogram of solvent is a lot more than 15.3 grams so the molality is going to be quite a bit higher so let's have a look the molality is moles of sulfuric acid per kilogram we have 18.4 moles of sulfuric acid for every 15.3 grams so in a kilogram we have 18.4 moles times the ratio a kilogram, 1,000 grams over 15.3 grams and we cancel out the units and we have 1,202 moles and therefore the solution is 1,200 molals I think that number is in fact quite a bit too high I think if you look up the literature you'll find it's more like 500 molals and I think that's down to me calling it 18.4 molars and the exact density that I used but in any case it's got a lot higher molality than molarity and that's always true if you have a concentrated solution the molarity is much lower than the molality on the other hand if you have a dilute solution if you have something let's say in water usually they have close to the same numerical value so let's convert concentrations again and let's do something that we might find at a hospital people have made very bad mistakes at hospitals by administering infants 1 molar heparin with one millimolar heparin which led to a lawsuit that you may have heard about you're supposed to dilute it you forget or you don't know what the units mean you blast this little kid with way a thousand times too much of something and even if something's a medicine a thousand times too much is usually not a good idea anything and you can have an adverse reaction and then if you fail to understand why you're getting an adverse reaction and deal with it you can have a death on your hands and you did it by being careless so let's take a 2 weight percent chemists don't like this 2 weight percent sodium chloride in water and I'm going to use that let's use a patient patient's in distress I'm going to puncture a vein I'm going to string up this bag like you've seen on TV and drip in some extra fluid so that they don't go into shock the question is what's the molality and molarity of this solution and would it be safe to use on an accident victim and I found out that the density of 2 weight percent saline is 1.0143 grams per cubic centimeter that's pretty close to the density of water usually that means if it's dilute like this that the molarity and the molality will be similar so let's have a look and let's see what they are oh, 2 weight percent means 2 grams of sodium chloride in 100 grams of the whole thing put together 100 grams of solution total amount of material and that means that it's got 98 grams of water 2 grams of sodium chloride at 58.44 grams per mol gives this number of mols a tiny number of mols a leader of this solution based on the density is going to be 10014.3 grams of solution and that's going to contain .02 times 10014.3 because this is the weight percent or 20.286 grams of sodium chloride I don't believe 5 digits here I'm not saying I believe that you can't measure usually things to 5 digits except voltage if you're lucky but this is an intermediate result in the calculation so I'm just keeping all the digits and I'm going to round it off at the end when I'm done this amount of grams of sodium chloride is .3471 mols or .35 molar usually we would call this 350 millimolar because we just like to use numbers that are bigger than one are kind of easier for us to understand as long as they aren't too big so it's the the concentration in molarity of sodium chloride in that 2 weight percent solution the rest now to figure out the molality I want to figure out how many grams of water is in the leader of solution and I've got 20.286 grams of sodium chloride and I know how much the leader of solution what mass it has 1014.3 so I just subtract them and therefore in that I've got 994 .1014 grams of water and therefore in 1000 grams of solvent we have this number of mols per 994.014 grams times 1000 and I should have put the grams in here but it didn't fit across the grams in and then I mentally cancelled the molality and I end up with .3492 mols the molality is 350 millimolar to 2 digits so the molarity is 350 millimolar and the molality is 350 millimolar okay close enough blood is around 150 millimolar salt that's why it tastes salty also tastes bad I guess unless you're a vampire if you transfuse the patient with this solution you will kill them you'll finish the job they got in a car wreck they're on their last legs fear not I'll save you wait a second what's in the bag forget it it's saline there are different concentrations of saline and they're used for different things so if you grab the wrong one that's it you just finish the job on that guy seawater is 3.5 weight percent salt and that's why if you try to drink seawater because you're thirsty you die you actually get more and more dehydrated as you go and you expire but the urge to drink something liquid that's cold if you're thirsty can be overwhelming I don't know how thirsty you've ever been but I've been extremely thirsty and I can tell you that some dirty puddle of water starts looking pretty good under those conditions you would never ever drink it normally but if you really think that you're going to die and your tongue is a dry lizard hiding out in a cave and turning colors and you're getting hot and cold and you've stopped sweating 3 hours ago that water looks pretty good and so if you get marooned on a sailboat in the ocean sometimes sailors would succumb to the urge to start drinking and then it's kind of like a siphon drinking something you tend to keep drinking it and then they would die often okay let's talk about this term ideal solutions we had an ideal gas an ideal gas was one that followed the ideal gas law and real gases were sort of close and if we have very similar molecules like two alcohols methanol and ethanol and we mix them together they'll mix in all proportion and they'll make quite a good solution that will approximate an ideal solution and likewise if we mix toluene and benzene that will also be quite an ideal solution what does it mean that it'll be an ideal solution well first it has to mix in all proportions from 1% of this to 99% of this if I've got two things they always mix entirely I never get anything sitting on the bottom I never get two layers second of all when I measure the vapor pressure of each thing they follow exactly what the mole fraction so this is why the theoreticians like this the vapor pressure of each thing in the solution follows exactly the mole fraction of the stuff in the solution times the vapor pressure of the pure liquid if I measure the vapor pressure of pure water at some temperature to be 400 tor and I have an ideal solution and the water has a mole fraction of 0.9 then the vapor pressure of water will be 360 tor right on and it'll follow if it's 0.8 it'll be 320 and so forth all the way down to 0 and this proportionality between the mole fraction and the vapor pressure is called Raoult's law and Raoult's law if all the components all the chemicals in a solution follow Raoult's law the solution is called ideal that doesn't happen that often but it does happen from time to time if you pour something into a solvent and the temperature changes it either gets colder or hotter no way is it going to be an ideal solution going to be way off but that's one clue if you pour something in and it gets really hot a lot's going on they're interacting and so they aren't going to follow Raoult's law of course if you pour in two beakers and they're in fact the same thing that's an ideal solution because it follows it perfectly and they don't really exist but real solutions are sometimes pretty close sometimes they're far off but sometimes they can be pretty close so let's have a look then at these ideal solutions in terms of the math so again we use chi for the mole fraction the pressure of the solution the vapor pressure and this is at a given temperature is equal to just the mole fraction of whatever the first thing is times whatever the vapor pressure of that pure thing is things have different vapor pressure and then the mole fraction of the second one times the vapor pressure of the second one and if I only have two things then the mole fractions have to add up to one so I can substitute for the mole fraction of the second one one minus the mole fraction of the first one and if I rearrange that I see that the vapor pressure of the solution is the vapor pressure of one of them plus the mole fraction of the other times the difference and that's I like that because I understand that if chi one is varying between zero and one which it does because it's mole fraction so now I can make a plot I start with chi one equals zero and it's the vapor pressure of the other stuff because it's a hundred percent the other stuff and then as I increase the amount the percentage of component one the vapor pressure heads finally when it's one the vapor pressure is chi one of the pure stuff and this is a straight line and therefore if I plot it and I look at the vapor pressure and it's always a straight line like that then I know it's following Raoult's law when it curves then it's deviating from Raoult's law and that's just the same as with gases when we keep doubling the pressure the volume can't keep having all the way down because the volume can't go to zero and so it starts curving same thing here alrighty well let's have a look the solvent if you've got a ton of solvent in solute then the solvent usually follows Raoult's law that's the normal assumption you just say look the solvent doesn't even have much of this other junk in it it's probably going to be pretty close to the pure solvent and it's just less of it and so it's going to have less of a vapor pressure of itself and that's because I think any of the solute most of the solvent molecules are just seeing themselves if I have one micro molar of something and I'm there as a solvent molecule I look around I can't even see the solute he's way over there I'm surrounded by just my same old buddies the solvent and so I behave the same if I'm going to go up I go up but the solute looks around the solute is the black sheep and a herd of white sheep he's looking around and saying hey this environment is totally different than if all these molecules were solute so oftentimes the solute doesn't follow Raoult's law even remotely so I've said that here the solute whatever the minor component is you have to be careful because if you have two liquids it's hard to decide who's the solvent and solute but if it's dilute the dilute one's the solute even if it's another liquid that's the convention it won't follow Raoult's law but it does follow a line the problem is the line is different so instead of having a vapor pressure the dilute one instead of being proportional to its mole fraction which is tiny times the vapor pressure of the pure solute it increases linearly but it's not this value of pressure it's this value k2 which has the units of pressure because mole fraction has no units at all but the value of this pressure can be bigger or smaller and sometimes by a lot compared to this one and that's just because the solute may not be welcome the solvent you throw the solute in the solute dissolves but the solvent doesn't like it the solvent saying hey why don't you just get out of here it tends to push it up with a little bit higher force than if they were all solute the value of k is higher or it could be that the solvent loves the solute and says under no circumstances leave I'm so tired of these other solvent guys here thanks goodness for a breath of fresh air stay down here and interact with us and then the vapor pressure is lower than what you would have predicted by an ideal solution and then the value of k2 is again a pressure but it's a lower pressure than the vapor pressure of the pure solute and both cases occur and k2 is given a name it's called the Henry's constant and this law even for a minor component that the pressure changes linearly with mole fraction but just with the wrong slope is called Henry's law and when two molecules are vastly different one of them can hydrogen bond the other can't things like that one of them has a lot of oily greasy stuff and the other one doesn't then the solute will tend to follow Henry's law but not Raoult's law and as I just said earlier k2 can either be larger or smaller depending whether the solute hates or loves the solvent gases follow Henry's law when they dissolve in liquids for example CO2 dissolves in water how we can make carbonated water and of course when you open a soda it's under pressure and it goes poof because some of the excess gas now sneaks out and then you have dissolved CO2 which gives you the bubbles which people like and you can make your own with CO2 and save a ton of money but more to the point this kind of calculation is ever so important if you start to argue about how much CO2 is going to dissolve in the ocean the ocean is a lot of water but we're putting in a lot of millions of tons of CO2 you might say well maybe we could get out of the jam if a lot of the CO2 dissolves in the ocean and some of it will but if you want to figure out how much you have to know the temperature of the ocean you have to know the Henry's law constant for CO2 and brine you have to know the partial pressure of CO2 in the atmosphere you have to know how much CO2 is already dissolved in the ocean because if it's up to the top already no more is going to go in and things like that and in fact we will dissolve more CO2 in the ocean most likely and that's already been picked up because CO2 makes the water solution acidic we get carbonic acid and in cases where the ocean becomes acidic the calcium carbonate structures that many kinds of shells and coral reefs use to build up their structure dissolves and when that dissolves all the small guys who are hiding out there are out in the open they're gone and then after they're gone the big guys look around and say hey there's nothing to eat and then they're gone and you can get a crash in the population when they first came to fish cod on the east coast in the Atlantic there were so many cod in the ocean that they just said many times you could never possibly exhaust them you just put a net in and you just get thousands of them they're just everywhere and that's what they did and now there's none there there wasn't an infinite supply it was quite finite lots of our food comes from the ocean and so we don't want to one way or another so we don't want to really pH the ocean at some new value and try that experiment on a global scale and then find out we don't get the result we want which could be far less food production so while that may you may think well that will not be so bad for global warming it could be very bad for global eating and if you look at where people go fishing now they're fishing the south pole they're pulling up all kinds of fish that you would never eat before and they grind them up and they call them I don't know crab or faux crab they serve them up to you and they're now fishing those guys out too so it may be that there won't be any fish at all as is the case for some kinds of tuna so here's a mixture of ethanol and water chemists love mixtures of ethanol and water here is the mole fraction of ethanol here the mole fraction of ethanol is zero it's all water and as I increase the mole fraction of ethanol the vapor pressure of ethanol goes on a straight line this is pretty straight but the slope is not this is the vapor pressure of pure ethanol the slope is higher in other words the water is pushing out the ethanol more pure ethanol and that's why if somebody has a drink your mom can tell because the alcohol comes out on your breath on a much higher level than you would expect and the human nose can pick that up immediately especially parents they have a nose if you look at water in pure ethanol it's the same thing pure ethanol says to water get out go on we're happy here we don't want you cluttering around we've got our interactions here with this hydrophobic methyl group get out and so the water when it's in almost pure ethanol follows a law but the apparent vapor pressure this is the Henry's constant the slope near here and it's way higher than the pressure of pure water which is on this on the other hand when we look at the pressure of water when it's almost pure water the mole fraction of ethanol is zero it follows Raoult's law it's linear up to pretty good concentration and it goes right down to zero so it's following that line likewise when it's pure ethanol it follows this line here that goes down here and it has the right slope here if the solution were ideal we could add up these lines and we'd get this line and this is Raoult's law if ethanol and water followed Raoult's law exactly they'd always be on that line but it shows a higher amount of total vapor pressure and they dislike each other and so we get that funny curve and that's the signature of a non-ideal solution this solution is not very non-ideal but it's kind of a good textbook example you can mix them in any amount but it starts deviating from the ideal behavior now we can use this because ethanol has a higher pressure than water so if we take a mixture of ethanol and water and we heat it the vapor is going to have more ethanol in it than water and the Scots figured this out a long time ago so they would take stuff that they had fermented up which would taste awful and they would distill it and they would make distilled spirits which were much much stronger and very habit forming and they'd come out with this stuff that's a clear stuff that's the worst stuff you've ever tasted unless you've tasted moonshine from Tennessee or hand sanitizer or whatever the latest trend might be if you want to look desperate distilling hand sanitizer that'll do it but then what they do is they take that distilled liquor and they put it in some oak barrels and they just let it sit around 20 years and you get some very slow chemical reactions and all the bad stuff the hard stuff goes away and you also leach some very aromatic compounds out of the wood usually you use an old barrel so you get some of the sherry flavor that turns into golden color and then it's quite delicious in small amounts but since it takes 20 years to do that scotch is expensive because how do you plan 20 years ahead what the demand will be you just make some and then if it turns out really good it becomes very expensive because everybody wants it to be made it's already been made so we can separate by fractional distillation these components and the ones with the highest vapor pressure will come off first and when they're distilling making distilled spirits they have a thermometer there and they don't let the temperature get above a certain level so when you do then you start boiling over all these awful things fusal oil and other things which will give you a whopping headache because they're much much more toxic than ethanol but if you're just moonshining you probably don't bother with a thermometer and you get what you get and if we keep heating it what will happen is we'll keep rising and we'll get less and less volatile things so if we've got a mixture of a thousand things we can try to separate them by distilling them we first make the temperature low and we collect something called a fraction we collect 5% and then the temperature goes higher otherwise nothing else comes out so we heat it up more then we get the next fraction distillation occasionally two things come off together and when that happens you've got something called an aziotrope if it doesn't change composition and that's how they denature alcohol they add something to the ethanol so that if you try to distill it the something they add comes off with it and the something they add is usually poisonous gasoline or something small amounts and then you don't have to pay the tax but you have to pay on drinks because you can't drink it same thing with listerine you put in a ton of thymol and eucalyptol you can't drink it it'll be very unpleasant in the petroleum industry this thing on just a massive scale these columns they have are gigantic and the amount of energy that they use to do the fractional distillation is just absolutely staggering in comes the crude oil 10 million barrels a day all of it gets boiled up by some means that is a lot of energy going into that so we heat that stuff up and we get various fractions here we get diesel oil here we get kerosene that's jet fuel here we get gasoline for cars here we get naptha that's for the fine chemical industry and then at the top they tend not to like because they tend not to like to handle gases and so the gases sometimes they just burn them off just flare them off total waste not worth enough money to hassle keeping it which is astounding but it's been true for a long time if you go up to montana north dakota now where they're drilling drilling drilling all these wells they're just flaring off the natural gas the methane just big flames sitting there doing nothing going up into the sky adding a fair amount of CO2 and then you get some real bad actors down here that are really heavy down here and this you use for ships the so-called bunker fuel and these really big ones as faultane those are like the bitumen in the pitch straw the blackest pitch that's what you use to pave the road you separate all that out but for every barrel of oil you boil up the whole thing how do you do that with another barrel of oil or with this gasoline you've got or whatever or these days you might use natural gas but still all that is producing a lot of CO2 and then after you take the oil and you fractionally distill it and you do all this stuff you then burn all of it too or the vast majority so the CO2 just goes up up up up up okay now if we add a non-volatile solute like sucrose or sodium chloride by non-volatile I mean that at room temperature the vapor pressure is so low that you can't measure it then we lower the vapor pressure of the water or the other solvent according to Raoult's law and the lower vapor pressure of the solution the other thing contributes no pressure but when I put it in I lower the mole fraction of the solvent so now the solvent has lower vapor pressure what that means is that vapor pressure up to one atmosphere I've got to go to a higher temperature because where it was one atmosphere when I throw in this solute it's now lower and that means it boils at a higher temperature and that's called boiling point elevation if I add salt to water or sugar to water and I add quite a bit it's going to boil at a higher temperature then just pure water the boiling point is elevated it's always elevated if the solute is non-volatile the kind of solute doesn't matter much just has to do with lowering the temperature we'll see that we can measure the molar mass this way by just changing the boiling point if we add a solute like sucrose or sodium chloride to a solvent like water and this, these guys sucrose and sodium chloride cannot dissolve in ice that seems like a kind of a funny thing to say because these molecules dissolve in water waters disordered as a liquid but when water starts forming a crystal ice and locking together it keeps pushing the sucrose or the salt out into the solution locks together and the ice you get from salt water is still pure therefore if you freeze ocean water slowly you can get fresh water from it so the icebergs are all fresh water they could be ocean water that froze slowly but there's no salt in the iceberg they're salt on the surface so far that's not a commercially viable option because the problem is washing all the salt off all the ice cubes you get and hassling around with that but it's theoretically interesting and if somebody works on it, it could be useful it's a lot easier to freeze water energy wise if you're in a cold place than it is to boil the water and then desalinate it by boiling all the water if you want an energy intensive thing get a couple of cubic miles of sea water and boil it all of it until you boil it all over and you've got nothing left but salt that's going to take a ton of energy to do that and the lower vapor pressure of the solution because the ice is pure the ice has the same vapor pressure normally you don't think of vapor pressure, ice, water what is he talking about but consider it this way let's physically separate the solution of salt and the ice cubes that are floating in it in two beakers and put them separate under an evacuated bell jar they're separate anyway when they're in there because the ice is pure but let's do it this way the ice has a certain vapor pressure it's equal to the vapor pressure of pure water right at zero that's why at zero the ice doesn't melt or disappear over to the water and the water doesn't freeze the solution now has a lower vapor pressure so that means that the ice actually is pushing out more water molecules than the solution and that means that the ice ends up going over here or it's got a lower melting point so it's going to be lower than just the pure solution so it's going to reach that point it's called freezing point depression now how does it work well I don't know if you've had this equation I think you may have but it's delta G named after Gibbs it's the Gibbs function that tells you what you're going to get at equilibrium and if we say that the free energy is equal to the enthalpy minus the temperature times the entropy then if we plot G versus T and H and S don't depend too much on temperature which they often don't then the plot of G versus T should be a straight line with slope of minus S S is the entropy that's the lowest for a solid so the slope is the least negative for a solid the higher for a liquid the slope is more negative S is very high for a gas the slope is extremely negative well suppose I plot the slopes for these three phases and have a look at what happens well here it is the system rolls downhill just like a ball very cold temperatures it's a solid as I heat it up the Gibbs function of the solid goes down down down down down down down and because the Gibbs function of the solid is lower than the liquid or gas it's solid because it already fell down to this level but when it reaches this point here it's going to melt because now the solid is going this way and the material says hey I can lower my Gibbs function much faster by turning into a liquid which it does until here and then it intersects the gas and then after that it says wow down I go I'm a gas and these two points are one it freezes and one it boils if I'm following it heating up now if I have a non-volatile solute it doesn't change if it doesn't dissolve in the solid it doesn't change the solid but it does lower the G of the liquid because the G is related to the pressure remember I told you pressure was energy per unit volume so it has something to do with how much oomph it has and it has less oomph when it's a solution and so what happens then is the liquid moves down and for simplicity I've kept it the same slope the solid doesn't change because the solute doesn't dissolve in the solid if the solid moved down by the same amount there wouldn't be any change it would be here again and the gas doesn't change because the solute doesn't go into the gas it's non-volatile and what we see then is now the solid to liquid it hits this red line and then it says wow I'm going to turn into a liquid but it doesn't hit this black line until higher so there's a lower freezing point and it's usually bigger because these are less of a slope and a higher boiling point but it's only a little higher and therefore usually if you want to use this a colligative property you focus on the freezing point you don't like putting your molecules in boiling solvents because your molecule may react it may fall apart or something bad may happen to it but usually if you're putting it in freezing it's easier to see and you can measure the freezing point by cooling it and then seeing when it suddenly levels off and you can get that quite accurately and then you can tell gee, the freezing point is lowered by a certain amount now we can see this effect of the lower vapor pressure of a solution in this practice problem let's consider this setup we have a bell jar there's nothing in it it's vacuum no air and we have a solution that's 10 millimolar sodium chloride and we have a solution that's pure water here and we have them in a thermostat at constant temperature thermostat's important because we may need to supply some heat energy for things to happen this is not insulated the question is what do we see at equilibrium who wants to take a guess exactly right all the pure water will magically end up over here and if it would happen quickly this would be a great magic trick since you can't see the salt but unfortunately it happens quite, quite slowly because we have to wait for the darn water molecules to come up here and fiddle around and come over here but the pressure pushing up here and so if I'm pushing on something and you're pushing me and I'm pushing you and I'm pushing harder you go backwards that's what happens and so all the water ends up over here and you end up then with 0.05 molar water in the other beaker and you can see I've tried to draw something here where on this side there's an occasional guy here and what it means is there's less guys coming up off the surface maybe this guy attracts them at the last minute and says don't go, don't go I'll change my way it's like people, they wait until it's too late bye and then these guys are giving more because there's nobody here saying anything and so over time more comes out of the right on the left over there and so here's what we get I've never tried the experiment this particular problem is at least 35 years old because it was on an AP test I took with a slide rule it's that old so this is an oldie but goodie but I've never tried the experiment I've always wanted to try it and just get two beakers and then also see how long it takes but I never had the time to do it okay the change in boiling point or freezing point depends on the kind of solvent and the concentration of the solute in molal but it doesn't depend on the kind of solute at least not much so we write this equation the change in boiling point is equal to this factor I times the boiling point what Chan calls the boiling point elevation constant times the molality and I is a factor that means if you've got sodium chloride you make two particles instead of one so for sodium chloride I is two because you get twice as many particles and it has to do with the number of particles in the thing the correct name for KB is the oblioscopic constant and after you say that a few times you'll see why they changed the name to the boiling point elevation constant but anyway that will clear your sinuses out and the factor I takes into account how many moles of particles the solute makes not the same for every kind of material obviously freezing point depression is the same thing it depends on the kind of solvent and the total number of solute particles and we write the same kind of equation exactly the same and this constant K only depends on the solvent when you're looking up K for boiling or freezing just think what's boiling or freezing mostly the solvent and so that's all it depends on you list it for that and the constant KF is called the cryoscopic constant may have heard of cryopreservation you get a fatal disease and some guy offers to freeze you in liquid nitrogen and then they're going to warm you back up when the disease has got a cure and that won't work you just die yeah and in fact a guy in Arizona was doing this in his garage he had at least six or eight suckers who meet frozen and their relatives would come by and check out the giant thing full of liquid nitrogen and one time he ran low on liquid nitrogen they missed the delivery and he's going oh god what am I going to do and so he just let one of the guys warm up and when the relatives weren't there and then he puts in more liquid nitrogen later now you're really dead if there weren't any questions the other guys went to court some of the relatives of the frozen man and said you know it's costing too much to keep feeding the old guy liquid nitrogen judge can't you declare him dead legally dead which the judge did he said look meets all the legal requirements I'm a lawyer the guy's dead and so they turned him off and so it's not a good option to freeze yourself unless you can somehow control what's going to happen after which pretty much you cannot okay for sodium chloride i is two for sucrose one and four calcium nitrate where we'd get three ions if it all dissolves and dissociates i would be three pretty simple freezing points pretty easy to measure and if we can measure the freezing point depression we can tell what the molar mass of a pure material is if we can find something it dissolves in and measure the change in freezing point which is dead easy to do we can figure out what it is and so let's go to a crime scene well not much of a crime scene forensic chemist has given a white powder half a gram of it dissolves in eight grams of benzene the freezing point is 3.9 degrees celsius could the powder be cocaine that's the question okay lots of things are white powders let's have a look you can kind of see why cocaine might dissolve in benzene because they both have these benzene type structure here and then cocaine has this very interesting structure here that the plant makes usually plants make these compounds not for fun they say why does the plant make all these things they make these things to keep the bugs off them because if you're a plant you can't move and these bugs are crawling all over you and they're drilling in and if you're going to survive you have to use chemical warfare you look at eucalyptus you start chewing the eucalyptus leaves ahh it kind of burns my mouth well it burns the insect's mouth too and that means they stop eating they don't go wild you plant lettuce they go wild and so you have to do the chemical warfare because otherwise they'll completely decimate your crops or you have to do something you can pick them off by hand that gets old because they're relentless and they come at night and slugs crawl over everything and wreck it so usually you spray and then the question is that has an environmental impact but presumably this has been made to keep off some bug maybe instead of burning the bugs mouth with eucalyptol the bug eats some of the coca leaf and then it says what are you trying to do here and just kind of flies off doesn't eat anymore and the plant survives who knows there are a lot of different strategies plants use well if we look up the molar mass of this molecule it's 303.353 grams per mole and if we look up the freezing point of benzene it's 5.5 degrees C little bit higher a little bit higher than water and the cryoscopic constant is 5.12 degrees C per molal that's how the units have to work out so let's have a look then whether it can work if the powder were 100% cocaine which is the real problem here then the freezing point should be the cryoscopic constant times the 0.5 grams convert to moles and then divide by 0.008 kilograms which is 8 grams and the freezing point depression should be 1.55 degrees C the observed freezing point depression is 1.055 the observed freezing point depression is 1.6 way off so our conclusion is it's impossible that that's 100% cocaine but sadly hardly anything is going to be 100% of the most valuable compounds usually some other things have been added and you don't know then if the other things are lowering the boiling by the freezing point by a different amount in fact what you have to do with the sample first is separate it using chromatography or other means until you get things that you see are pure you can just take a tiny spot of the material you can put it on a silica sheet and you can put solvent the solvent percolates through and if it's one material you have one spot that moves more than one material you get different spots that's called thin layer chromatography you only need a tiny amount to see that it's not pure yes because benzene freezes at 5.5 and what the forensic chemist found is when they put this stuff in it froze at 3.9 and that's 1.6 difference so what I'm saying is it can't be that molar mass all you know is it's not that molar mass you don't know it's not some other drug okay but it's not that drug you would then try some other ones okay let's do one more thing then we'll quit by the way you'd never do it this way if you go to a crime lab they don't they have mass spectrometers and they'll tell you or the Olympic testing when they find one extra molecule of testosterone that's really interesting chemistry sort of cloak and dagger find out who's cheating the short answer is everybody's cheating in sports they just haven't been caught okay if we have a semi permeable membrane and we can let through some molecules like water while excluding other ones like charged ions you might think how do you do that but in fact that's what our cells do all the time they let through potassium into the interior of the cell and the sodium stays in the plasma of your blood so there are membranes well if we have an artificial membrane like that the G value is lower for water with some solute because we saw its vapor pressure is lower therefore if we trap a solution behind a membrane and put pure water on the other side then the pressure can be huge and the water will tend to rush in and swell the stuff up and you can see this effect on some people who eat too much salt look very puffy for a while and their kidneys work overtime get rid of all that stuff back to normal another bag of Doritos or something and you're back to where you started don't eat salt it's not a good idea try to steer clear of it and you'll get just the right amount of smoking you don't need to worry about having a minimum amount to be healthy and this pressure then we'll pick up this next time when we talk about desalinization of water by reverse osmosis