so when we brought up the concept or the idea of a non-volatile solute what we talked about is that it lowers the vapor pressure of a solvent so that the solution always has a lower vapor pressure than the pure solvent does what that looks like on a phase diagram is shown on the right and here we have the pure solvent this is a vapor pressure curve this is a freezing curve the freezing the solid liquid transition and this is the curve for sublimation and again this is the triple point where the three meet well if you lower the vapor pressure of a liquid so let's say this is its normal boiling point when it's a pure solvent then when you add a non-volatile solute we drop the vapor pressure of that substance and then in order to get it to boil what we have to do is we have to increase the temperature of the solution again so you can take a solution actually that's boiling throw a solid in it that will dissolve it will stop boiling because now its boiling point is higher than it was before and then you'll have to increase the temperature to get it to boil again now notice what happens at the other end of the curve since you're lowering the vapor pressure at all points on the curve it intersects the transition between the solid and gas at a lower temperature this also causes the liquid solid transition to shift backwards as well so what we end up getting is a boiling point elevation on one side that's a tb down here and a freezing point depression going the other way now of course it's called an elevation because it's going up and this is a depression because it's going down turns out both of these can be calculated using the same form of an equation so to calculate the boiling point elevation the equation is given as tb or sometimes it's delta tb okay is equal to k b times m where m is the molality of the solute particles now if you're using ionic compounds right then each ionic compound will break down and give you more okay more ions and so we use a slightly different form of the equation this is known as a van't hoff coefficient and for sodium chloride that values very close to two because sodium chloride gives us two ions for every mole of substance that dissolves and then for the freezing point depression tf that's usually calculated as kb kf times m where again m is molality now kb and kf are boiling point elevation constants and freezing point depression constants and this leads us into something called colligative properties boiling point elevation and freezing point depression are part of a group of properties known as colligative prop so colligative properties don't depend on what the particle's identity is it only depends on the number of particles that are produced so and another important feature is that the properties are solvent properties since the identity of the particle doesn't matter it turns out the properties that we're looking at are dependent on the solvent very much for the same reason when we were looking at raoult's law for non-volatile solutes right it's really the mole fractions of the solvent that matter in the property and it doesn't really matter what the other substances are in the solution it just matters what the mole fraction of the solvent is so colligative properties are very much like those now and when we talk about these strictly speaking we're talking about non-volatile solutes but if you have solvents with very low vapor pressure or solutes that have very low vapor pressure it can behave the same way now if the solid is like sodium chloride the one thing about ionic compounds that's important to understand is that they tend to produce more particles than their moles of substance for example one mole of sodium chloride theoretically can produce two particles and magnesium bromide could theoretically produce three particles as it turns out this has been tabulated how how much of an effect ionic compounds can have has been tabulated and they're called vant hoff factors and they're usually done for relatively dilute solution and we just assume they apply roughly to the calculations that we're doing so it says van't hoff factors at .05 molal right concentrations of aqueous solution non-electrolyte that would be molecular compounds have i values of one so when we're doing the for example delta t is equal to i times k b times m right if the substance is molecular we treat i as one but for sodium chloride theoretically it's two but because of ion pairing and other factors it ends up being about 1.9 magnesium sulfate has larger larger charge magnitudes plus two and minus two and that actually has an influence on its van't hoff factor so instead of being two particles being produced it behaves more like 1.3 so there are deviations dependent on charge for the most part in this class unless you're given the vantov coefficient then we're going to assume it is exactly what it says it is for the formula now just a couple other examples it can deviate quite a bit here's four particles and you get 3.4 right three particles 2.6 in general though compounds like sodium chloride are very close to what we would expect theoretically so how would you calculate a boiling point or a freezing point change as a result of having a non-volatile solute so suppose you had a 0.5 molal solution of let's say glucose so that's c6h12o6 first thing you have to recognize is that this is a molecular compound then you would assume that for example the delta t or sorry the t the t boiling let's say the boiling point elevation would be i right which times k b times m that would be equal to one now the value for kb we would look it up for if it's an aqueous solution we would look it up for water so water k b is here is 0.512 degrees celsius per molality unit so this is 0.512 degrees celsius per molality unit and then we would multiply that by 0.5 m so that comes out to be 0.256 degrees celsius so what's the boiling point of the solution we you assume that it's an elevation because it's a boiling point and then what you do is you add this value to the boiling point of the pure solvent so in this case we would go to water and here's its pure boiling point 100 degrees celsius and so the t the boiling point will be 0.256 degrees celsius plus degrees celsius and that would give me approximately 100.3 rounding to the correct number of sig figs like that here's another example this is calculate the freezing point of a 1.74 molal solution of naphthalene in carbon tetrachloride so again naphthalene is a molecular compound so we're going to have tf is going to be i right times kf times m and then for i if it's molecular we give it a value of one and then kf we have to look up the freezing point depression constant for carbon tetrachloride so this would be carbon ccl4 and we would look it up on our chart and it has a freezing point depression constant of 29.9 degrees celsius so this becomes sorry 29.9 degrees celsius per molality unit okay and our molality is 1.74 mole so that ends up being 52.0 degrees celsius because the molality units will cancel now so what's the freezing point of carbon tetrachloride to do that what you have to do is you have to go back and look at the normal freezing point for carbon tetrachloride so it's minus 63.5 degrees celsius so this is a depression right so it's 63.5 degrees celsius minus minus sorry 56 minus 52 0.0 degrees celsius minus 115.5 degrees celsius or negative 115.5 degrees celsius so in order to calculate the freezing point always remember it's a depression so you take the current freezing point and you're going to subtract that off the freezing point uh the freezing point depression off of the actual normal freezing point to get the freezing point to pre the depressed freezing point hopefully i said that right so this is your new freezing point for that solution so the last of the colligative properties that we talk about is osmosis and the idea of what's called an osmotic pressure so osmosis uh has a kind of a weird definition it's it's very uh content content uh context specific and the idea is this osmosis is the movement of water so this particularly has to do with water through a semi-permeable membrane so let's look at what normal osmosis does so this is a illustration from your book and they have this tube it's called a youtube of all things and first you think of it as being filled with water right and then on this side we've added solute particles again non-volatile solute particles and what happens when you allow the water to flow well the movement of the water through this membrane okay causes water to go from the right hand side to the left-hand side now the nature of the semi-permeable membrane is that in principle it only allows water to pass that's why they call it semi-permeable and in essence what the solution what the water is trying to do is to dilute this solution and increase the entropy on this side of the solution because we think about particles in us in a in a liquid right what what that creates is a situation of entropy and it turns out in nature there's this natural tendency to go to higher entropy so if you think about it like this right then the water wants to move from the side with no solute to the side with solute it'll actually move it in enough that you'll see a difference between the levels of water on one side and the other the pressure that's represented by this difference in liquid from this side to this side is known as the osmotic pressure and it has an unusual equation for calculating it the equation look maybe familiar but it has to do with p right is equal to n over v times r t and what is that equation right that's the ideal gas law most commonly though when it's referring to osmotic pressure what it's given as is pi this is the osmotic pressure is equal to big m that's molarity because we're talking about a solution so n over v is the molarity times rt again if it's molecular in nature then the there's a vant hoff factor that goes in front of here that's one but if it's ionic in nature then there's a vantov factor that goes there that's approximately 1.9 so if you can calculate the molarity of the solution you can calculate the osmotic pressure that can tell you in the ideal situation how high this side of the liquid will be relative to this side i have a couple little videos to illustrate osmotic pressure oh by the way this r right is just the ideal gas constant 0.08206 liters moles atmospheres kelvin when a solution and a pure solvent are separated by a semi-permeable membrane solvent can flow in both directions but the solute cannot here pure water moves into the solution working to bring both the solution and pure water to the same concentration this process osmosis continues until the flow of water across the membrane is balanced by the pressure exerted by the water column there's another short illustration a semi-permeable membrane blocks solute molecules but allows solvent molecules to pass in both directions the number of solvent molecules moving from the solution side to the pure solvent side is less than the number moving into the solution side resulting in a net flow into the solution so honestly this is a little bit of a misconception on how osmosis works they talk about the particles blocking one side or the other but what is true about it is there's more water molecules flowing from the side of low concentration to the site of high concentration so with that i'm going to move into a slightly sort of gray area for chemistry we're going to talk a little bit about how osmotic pressure can affect red blood cells so the there's a unit called the melee osmolal or milli osmolarity and it's the same as molarity except that when we're talking osmolarity we're typically talking about the molarity of solute particles that can affect the osmotic pressure so a term that goes along with osmolarity and osmotic pressure is the concept of hypertonic hypotonic and isotonic now now you'll often hear iso osmolar which is oftentimes uh believed to be the same molarity but really it means it has the same osmotic pressure and then that also gets equated with the term isotonic which means there's no net solution flow so what i've shown here i'm going to show you try to sort of tell you what the difference of these things are because it's not really clear so this is these are examples of red blood cells and these are normal red blood cells that means they're in an isotonic solution so what an isotonic solution is is a solution where the there is no net fluid flow from outside the cells to the insides of the cells in other words there's a rate of of liquid going in water and a rate of water coming out and they have the same tonicity then now if we change the outside solution and we make it hypertonic okay then what will happen is water flows from the inside of the cell to the side that's hypertonic this means this the cells shrink and they take on this unusual appearance now if you put it in a hypotonic solution then what it means is there's a net flow of water from the outside of the cell to the inside of the cell so the water is moving like this now when we say that then actually hypotonic and hypertonic and hypotonic are two sides of the same coin because when we say the water moves from inside the cell oops all right we'll try that again so when we say that water is moving from inside the cell to the outside of the cell it means that the inside of the cell is hypotonic relative to the outside and then when we say that it's hypertonic or when we say it's hypotonic on the outside really what that means is it's hypertonic on the inside and this actually doesn't have to do with the concentration of the solute just by itself for a lot of ionic compounds that can't cross the membrane of the cell these two terms this hypertonic and hypotonic and hyperosmotic and hypoismonic can be the same but it turns out there are a lot of organic materials that can cross the the cell membrane so if you have a hyper oz let's go back here so this is supposed to be isotonic right this is when water flow inside to outsides the same now if you make it iso osmotic that just means it has the same osmotic pressure but if you take a solution of glucose and remember glu glucose if you've had biology is the food of the cell right then what happens is all the glucose goes inside the cell and glucose solutions behave like pure water to the cell so even though it may have the same concentration of solute that is has the same osmolarity it doesn't have the same tonicity it means the glucose essentially doesn't exist on the outside because it moves to the inside carries water to with it and then gets consumed as a source of energy so it behaves like distilled water on the other hand uh an ion on the outside that doesn't penetrate the cell wall right it's osmolarity if it's i if it's iso-osmotic it's also isotonic in that case but glucose goes inside the cell membrane so you have to be a little bit careful the book talks about these two things hyper osmotic and hypertonic and hypoosmotic and hypotonic as being the same thing but in medicine and in like health areas these are not the same thing they're slightly different anyways i just thought i would bring that up and show you what it looks like right when you when you're looking at actual cells and these solutions are either hyper or hypotonic so we're going to calculate the osmotic pressure of a solution that contains 18.75 milligrams of hemoglobin at in 15 mils of solution at 25 degrees celsius and it says the molar mass hemoglobin is 6.5 times 10 to the fourth grams per mole so to begin with we know osmotic pressure right pi is equal to m times r times t and t is 298.15 kelvin now to get the molarity or r is 0.08206 liters atmospheres over moles times kelvin like that so i just need to calculate the molarity of the solution so to get moles right i have 18 this will be my moles i have 18.75 times 10 to the minus third grams that's the again substituting the definition of a milligram and then i know it's 6.5 times 10 to the fourth grams for every one mole that gives me 2.88 times let's see 10 to the minus 7 moles and then to get the molarity i have 2.88 times 10 to the minus seventh moles divided by 15.0 times 10 to the minus third liters and that gives me 1.9 2 times 10 to the minus fifth and that will be in moles per liter or molarity units and now all that's left to do is plug it into this equation so i have pi equal to and i'll just write it all out 1.92 times 10 to the minus molarity times 0.082061 liters times atmospheres over moles times kelvin times 298.15 kelvin that leaves one let's see what is it no 4.7 times 10 to the minus 4 atmospheres oftentimes this is calculated in millimeters of mercury or in tor and that's actually zero i'll write it above here zero point 8 torr like that either one of these depending on you the units that are in the back when you look it up this is actually one of the even number problems out of your book all right so again pretty straightforward strategy is similar to what you might expect you need to find the molarity which means you need to find the moles you're given grams or milligrams so we convert milligrams into grams and into moles using the given molar mass then we divide by the volume to give us some molarity and then plug it into the equation and finish your calculation