What is the vapor pressure in mm Hg of the following solu-tions, each of which contains a nonvolatile solute? The vapor pressure of water at 45.0 °C is 71.93 mm Hg. (b) A solution of 10.0 g of LiCl in 150.0 g of water at 45.0 °C, assuming complete dissociation

Video transcript

Hello. In this problem we are asked what is the vapor pressure in millimeters of mercury of a solution? 12 g of sodium fluoride dissolved in 200 g of water at 60 degrees Celsius. The sodium fluoride is a non volatile salute and completely dissolved in water, He told the vapor pressure of water at 6°C is 149 mm of mercury. So recall we have these collaborative properties which are changes in the property properties of the Sullivan. In our case that is water when solid particles. In our case, sodium fluoride is added. One of those collective properties is vapor pressure lowering rail slaw then describes the pressure of water above the solution where X is the mole fraction of water times the water vapor pressure for pure water. So collective properties also depend on the number of particles that are present. And in the case of our non volatile salute, sodium fluoride, it is going to disassociate in water, perform so do your minds and Floridians. So we have to take into account that for every one mole of sodium fluoride that we put in solution, we get two moles of particles. So to find the vapor pressure of the solution which is given by the vapor pressure of water, we have to find the mole fraction of water To find the mole fraction of water. We have to find the moles of the ions that are in solution as well as the moles of water. So let's begin with our ions. So we have 12 g sodium fluoride. Making use of its more mask from mass moles. one mole of sodium fluoride has a mass of .998g. And taking into account that the sodium fluoride will associate. one mole of sodium fluoride will produce two moles Lions, so one mole of sodium mine and one mole of Floridians. So this works out then to a total of 0.5716 moles of ions. Now determine the number of moles of water. We have 200 g of water using the molar mass of water, One mole of water has a mass of 18.015g. And when setting up all of these conversion factors, let's make sure that our units are canceling. So let's go back up to the sodium fluoride. You see that mass of sodium fluoride cancels and then the moles of sodium fluoride cancels and turning back to water, we have the massive water cancels and this works out to them 11.10 moles of water. Our total number of moles then is equal to the moles of our ions 0.5716. And our moles of water 11.10. This works out to 11.672 moles. The mole fraction of water then is found by taking moles of water 11.10 moles And divided by our total moles 11.672. And this works out to 0.951. So the vapor pressure of water above the solution is equal to the mole fraction of water, which is .951 times that Vapor pressure of pure water, which is 149 mm of mercury At 60°C. So the vapor pressure of water above the solution is a higher and 42 mm of mercury. So we go back up to the top, right? So we talked about then these collective properties are changes in the properties of the solvent. When we add solid particles and one of those collective properties is vapor pressure lowering. So when we add sodium fluoride, we expect the vapor pressure to be lowered. The Vapor Pressure of Pure Water is 149 mm of Mercury. And we found that the vapor pressure of water above the solution is millimeters of mercury. And so indeed the vapor pressure of water above the solution is lower than that above the pure water. And so the answer makes sense. In terms of the directionality, we should see that the vapor pressure is being lowered. Hope this helped. Thanks for watching