Vapor Pressure Lowering (Raoult's Law) - Video Tutorials & Practice Problems
Get help from an AI Tutor
Ask a question to get started.
Vapor Pressure is the pressure exerted by a gas at equilibrium with its liquid phase at a specific temperature in a closed system.
Understanding Raoult's Law & Vapor Pressure Lowering
1
concept
Vapor Pressure Lowering Concept 1
Video duration:
2m
Play a video:
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.
2
example
Vapor Pressure Lowering Example 1
Video duration:
2m
Play a video:
Calculate the vapor pressure in Tore of a solution containing 53.7 grams of Cadmium Nitrate, which has a molecular weight of 236 point 43 grams per mole, and 155 grams of water at 30 degrees Celsius. Here we're told the vapor pressure of pure water at this temperature is 131.8 towards. Alright. So first what we're gonna do is we're gonna calculate the moles of both cadmium nitrate as well as water. So we have 53.7 grams of cadmium nitrate, and we're told for every 1 mole of cadmium nitrate, we have 236.43 grams. So here, when we plug that in, that's gonna give me, moles of 0.22713 moles of cadmium nitrate. And then we're gonna do the same thing with water. We have 1 55 grams of water. For every one mole of water, its mass is 18.016 grams, So that equals 8.60346 moles of water. With that, we can now figure out the pressure of our solution. So pressure of solution equals the moles of our solvent, so 8 0.60346 moles of water divided by I is the number of ions your solute will break up into in solution. Cadmium nitrate is ionic, so it breaks up into 1 cadmium ion plus 2 nitrate ions. So that's a total of 3 ions, so I equals 3. So 3 times the moles of cadmium nitrate which is 0.2 2713 moles, plus the moles of water again. Okay. And then that's gonna be multiplied by our 131.8 tours. So when we do that we're gonna get as our final answer, 122 tours for our solution. Here, our answer has 3 sig figs because 53.7 is 3 sig figs and 155 also has 3 sig figs.
3
Problem
Problem
How many grams of glucose, C6H12O6, must be added to 515.0 g of water to give a solution with a vapor pressure of 13.2 torr at 20.0ºC? The vapor pressure of pure water at 20.0ºC is 17.5 torr.
A
9.54 × 102 g
B
1.68 × 103 g
C
5.29 g
D
9.31 g
4
Problem
Problem
Determine the vapor pressure lowering associated with 1.32 m C6H12O6 solution (MW:180.156 g/mol) at 25°C. The vapor pressure of pure water at 25°C is 23.8 torr.
A
0.553 torr
B
27.6 torr
C
23.2 torr
D
0.976 torr
5
Problem
Problem
The vapor pressure of water at 100.0ºC is 0.720 atm. Determine the mass percent of iron (II) chloride, FeCl2, needed to reduce its vapor pressure to 0.655 atm. (MW of FeCl2 is 126.756 g/mol)