14. Solutions
Vapor Pressure Lowering (Raoult's Law)
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Raoult's law deals with vapor pressure lowering. Here, we're gonna say that vapor pressure is the pressure exerted by a gas that is in equilibrium with its liquid phase at a specific temperature within a closed system. Now here, if we were to look at vapor pressure and Raoult's law, we're gonna say here vapor pressure is the pressure exerted by a gas at the surface of a liquid. So remember, this orange portion represents the surface of the liquid. We have gases already dissolved within my solution and we have gases above it. Remember, under vapor pressure we have both condensation and vaporization happening at the same rate. The vapor 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 custom to seeing mole fraction with solute on top. With vapor pressure, it's not that, it's solvent on top now, divided by I, remember I is your van't Hoff factor, and I is associated with our solute, so it'd be I times our moles of solute, plus moles of solvent. Now look at how it's written. The I and the moles of solute are what are multiplying each other, not the moles of solvent. Moles of solvent are off to the side. Alright? 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, 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 solute to solvent, so that'd be the pressure of our solution, because remember solute and solvent together equals a solution. Where we talked about I being the Van't Hoff factor, if you're not familiar with that variable, take a look back back on some of my videos dealing with the colligative properties, this being one of them. In the colligative property section, we talk about the Van't Hoff factor. Next, we have mole fraction of our solvent, which is xsolv, and then pressure of our pure solvent is pressure degree solvent here on the bottom. Now, here we're gonna 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 you should always keep in mind when dealing with different calculations with vapor pressure lowering.
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