 Okay, so just as we had for freezing point depression, we've now ended up with a variety of different expressions we can use to compute the boiling point elevation in a solution, depending on whether we want to use the activity or the mole fraction or the molality as our measure of how much solute is in the solution. And depending on how important it is that we use the simpler of these expressions, we have a variety of expressions we can use. So let's do another example of computing a boiling point elevation constant like we did for the freezing point elevation constant. But instead of doing it for water this time, let's choose a different solvent. So let's say that the solvent we're interested in is octane, n octane. So n octane is a solvent that has a boiling point of almost 400 Kelvin, a little higher than water. It has an enthalpy of vaporization of 34.98 kilojoules per mole. And its molar mass is 114 grams per mole. So that's enough information for us to compute the boiling point elevation constant so we can learn how much the boiling point is increasing per one molal of a solution that we prepare. So we can use this expression to tell us how to compute that boiling point elevation constant. We need the molar mass of the solvent and we're going to want to convert that from grams per mole into kilograms. We need the gas constant. We need the boiling point. So still in the numerator I'm going to multiply by this boiling point 398.81 Kelvin that gets squared. And that's all over the denominator of the enthalpy of vaporization 34,980 in units of joules per mole. And the units are going to work exactly like they did for the freezing point depression constant. We've converted grams to kilograms. One of these moles disappears, the joules disappear. One of these Kelvin disappears with this Kelvin and what we're left with is units of Kelvin kilogram per mole. And the numerical answer to that question if I multiply and divide these numbers is four sig figs I'm entitled to, so 4.317. So 4.317 Kelvin per kilogram per mole or 4.317 degrees Celsius per molal. It's an equivalent way to think about it. So that number is fairly large. I can change the boiling point of octane by four degrees Celsius for every one molal of solution that I prepare and that boiling point again gets elevated, gets increased by four degrees every time I increase the concentration by one molal. If we want to work a quick example with that number, actually before I do that I'll point out that this boiling point elevation constant clearly I've used values for octane. That value is going to be specific to octane. It would be different for every sol vent. So for example in H2O if we were to repeat this calculation using the enthalpy of vaporization for water and its boiling point and its molar mass we're going to get a different value. Water for example only increases its boiling point by half a degree Celsius per one molal of solution that I prepare. So octane has a significantly higher boiling point elevation constant. Its boiling point changes more when we increase the concentration. So that's mainly a property of the sol vent. So if we do want to do an example instead of giving you a molality and multiplying these two numbers to find the boiling point elevation let's do a slightly different flavor of example. Let's say I want you to prepare a solution that has a boiling point not of 398.81 but let's say 399.81. Let's say I want the boiling point to be elevated by one degree Celsius, one Kelvin. What concentration of solution would you prepare? That's almost as easy. The molality would be the change in the temperature, the boiling point divided by the boiling point elevation constant so if we want to change by one Kelvin and the boiling point elevation constant is about four. So one divided by four is about a quarter or more specifically I would need a concentration of 0.23 mol per kilogram in order to elevate the boiling point of octane by a full degree. Notice that I don't have to tell you what solute I'm using to elevate the boiling point of octane. It doesn't matter whether I dissolve sugar in octane or whether I dissolve a little bit of ethanol in the octane or a little bit of water in the octane, it doesn't matter what solute I try to dissolve in the octane, the concentration of the solute and all I need to know is the concentration not the identity of it. The amount of the boiling point elevation is the property of the solvent not the solute like with all colligative properties. So we've considered freezing point depression and boiling point elevation as two of our colligative properties so far as well as vapor pressure lowering so there's a fourth one that we'll consider next and that's the osmotic pressure.