The following phase diagram shows part of the liquid–vapor phase-transition boundaries for two solutions of equal con- centration, one containing a nonvolatile solute and the other containing a volatile solute whose vapor pressure at a given temperature is approximately half that of the pure solvent.

(d) Based on your drawing, what is the approximate normal boiling point of the pure solvent?

Video transcript

Hi everyone for this problem. It reads Consider the diagram below showing the vapor pressure curves for two solutions with equivalent concentrations, solution A contains a non volatile salute while solution B contains a volatile salute with a vapor pressure of around half of the vapor pressure of the pure solvent at a certain temperature, approximate the vapor pressure curve for the pure solvent and determine its normal boiling point. So we're going to approximate the vapor pressure curve for the pure solvent and we're going to determine its vapor or its normal boiling point. So in the question we're told we have two solutions. Right? We have a non volatile salute. We have a volatile salute and we have a solvent and a solution. Okay, so let's just define these two things and what those mean. Alright, so let's go ahead and first discuss of solvent plus a non volatile a solvent and a non volatile salute. Okay, so this is a collective property and because it's a collaborative property this is going to lead to significant vapor pressure lowering. However, if we have a solvent plus a volatile salute, this is not a collaborative property. And so there's more inter molecular forces of attraction which leads to lower interactions. And what that means is that it's not so significant vapor pressure lowering. So we'll just right here not significant vapor pressure lowering. Okay, so that's cut off. Let me just move that to the side so that's for our solvent and a non volatile salute and a volatile salute. Okay, so let's go ahead and say the same thing. But for solution instead of the solvent. So for a solution with a volatile saw you, this leads to higher vapor pressure and in this case this is the blue which is solution B. Now for a solution with non volatile saw you this has a lower Or this leads to a lower vapor pressure which is from our graph, the Red one which is solution A. Okay, So the vapor pressure of the pure solvent will be higher than solution A. And solution B. But the boiling point of the pure solvent will be lower and the vapor pressure of a liquid at its normal boiling point is 760 millimeters of my mercury which is equal to atmospheric pressure. So looking at it, the vapor, the vapor pressure for solution B is going to be about one half the vapor pressure of the pure solvent at a certain temperature, which we can see here is at about 380 of Mercury. So looking at the vapor pressure curve, our answer in terms of the boiling point, the boiling point is going to equal 60°C. Okay, so that is going to be our final answer for this problem. And that is it for this problem. I hope this was helpful