Vapor Pressure Lowering (Raoult's Law)

Roberts law deals with vapor pressure lowering here, we're going to say that vapor pressure is the pressure exerted by a gas that is an equilibrium with its liquid phase at a specific temperature within a closed system. Now here, If we were to look at bigger pressure and rolls law, we're gonna stay here. Vapor pressure is the pressure exerted by a gas at the surface of the liquid. So remember, this orange portion represents the surface of the liquid we have gas is already dissolved within my solution, and we have gasses above it. Remember, under a bigger pressure, we have both condensation and vaporization happening at the same rate. The favorite pressure is just the pressure that results from that on the surface of the water. Now, vapor pressure lowering comes with its own formula, and that is the pressure of our solution equals the mole fraction of our solvent. And since it's the mole fraction of our solvent here, that would be moles of solvent on top. We're accustomed to seeing mole fraction with solid on top with vapor pressure. It's not that it's solvent on top, now divided by I remember, I is your benefactor and I is associated with our salute. So I'd be I times r Moses salute plus moles of solvent. Now look at how it's written the I and the moles of solute or what are multiplying each other Not the moles of solvent solvent are off to the side. All right, And then here. So that's the mole fraction of solvent times the pressure of our pure solvent. And to show that it's pure, we put that little, uh, degree sign above. So here this is our formula for vapor pressure lowering. Now, if we look at all the variables, this equals the pressure after adding solid to solvent. So that be the pressure of our solution. Because remember, Sol Yoon, insolvent together equals a solution wherever you talked about driving the Van Hoff factor. If you're not familiar with that variable, take a look back on some of my videos dealing with the collective properties. This being one of them in the collective property section, we talk about the benefactor. Next, we have mole fraction of our solvent, which is X S O l V. And then pressure are pure solvent. Is pressure degree solvent here on the bottom now here, we're going to say, since adding solute lowers the vapor pressure, that means that the pressure of my solution is always less than the pressure of my pure solvent. So that's just a piece of information should always keep in mind when dealing with different calculations, with vapor pressure lowering.

calculate the vapor pressure in tour of a solution containing 53. g of cadmium nitrate, which has a molecular weight of 2 36.43 g per mole in 155 g of water at 30 degrees Celsius. Here we're told the vapor pressure of pure water at this temperature is 1 31.8 tours. Alright. So first, what we're gonna do is we're gonna calculate the moles of both kaboom nitrate as well as water. So we have 53.7 g of cadmium nitrate and we're told for every one mole of cadmium nitrate, We have to 36.43 gramps. So here, when we plug that in, that's going to give me a moles of .22713 moles of cadmium nitrate. And then we're gonna do the same thing with water. We have 155g of water For every one mole of water. Its mass is 18.016 gramps. So that equals 8.60346 moles of water. With that, we cannot figure out the pressure of our solution. So pressure of solution equals the moles of our solvent. So 8.60346 moles of water divided by I is the number of ions Your salute will break up into a solution. Kaboom nitrate is Ionic, so it breaks up into one cadmium ion plus two nitrate ions. So that's a total of three ions. So I equals three. So three times the moles of cadmium nitrate, which is .22713 moles plus the moles of water again. Okay. And that's gonna be multiplied by R 1318 tours. So when we do that, we're gonna get as our final answer tours for our solution here. Our answer has three sig figs because 53.7 is 36 figs and 55 also has three sig figs.