Which of the following solutions has the lowest vapor pres- sure? (Assume complete dissociation for ionic compounds.) (LO 13.10) (a) An aqueous solution of 0.25 m Ca1NO322 (b) An aqueous solution of 0.30 m sodium chloride (NaCl) (c) A mixture of 50 mL of water and 50 mL of ethanol 1CH3CH2OH2 (d) A mixture of 25 mL of water and 50 mL of ethanol 1CH3CH2OH2

Video transcript

Hello everyone today. We have the following problem determine the solution with the lowest vapor pressure, assuming that ionic compounds completely dissociate. So the first thing we wanna do is we want to define what vapor pressure is. So in the simplest terms vapor pressure is just the force that is exerted on a surface of a liquid. Buy a gas. And so we can say that when a song is dissolved in a solvent, the solute particles interfere with the evaporation, resulting in less vapor and thus less vapor pressure. And so we can have the relationship formed that the more solute particles that we have, the more solid that we have, the lower the vapor pressure. So if you look at a this is a mixture of 100 ml of water and 100 ml of isopropyl alcohol. And if we look at B as well well isopropyl alcohol is both a volatile salute and a non ionic compound. So we can say isopropyl alcohol is a volatile salute as well as being non ionic. There are no ions when isopropyl alcohol associates. And so this will either increase the vapor pressure or just evaporate. So because it is non ionic. So because it is non ionic this just means that it will not associate. So you can say answer choice C. Or answer choice A. Is going to be incorrect as well as be now looking at options. See we need to calculate the oz morality and so the osmolarity, abbreviated O. S. M. Is going to be good I or the Vantaa factor or the number of ions that are associated times the morality, which is lower case M. So For C we have an agreed solution of . moralities. Barium nitrate and then we have barium nitrate there. So barium nitrate is going to dissociate into three compounds. So our eye is going to be three. Let's see when barium nitrate associates, it's going to disassociate and to bury them two plus ions as well as to nitrate ions. So we said I is equal to three. And then we have the morality here which is 0.4 And that's gonna give us 1.2 oz moles of solute per kilogram solvent, If we look at D we're going to apply the same principle, we're going to solve for the oz morality, That's going to go to the Van Hoff factor. The number of ions associated times our morality. So for sodium bromide, that's just going to associate into sodium plus ions and bromide ions. So it's gonna associate into two ions. So we're gonna have to, for I and the normal reality given to us was 0.25, this is going to result in 0.5 oz moles. Now if we compare these two, we can see that and gives us the higher number of oz molds and therefore will provide us with the lowest vapor pressure. And so with that answer to a C. Is going to be our correct answer. Overall I hope that this helped. And until next time