What is the vapor pressure in mm Hg of the following solutions, each of which contains a nonvola

What is the vapor pressure in mm Hg of the following...

Key Concepts

Raoult's Law

Raoult's Law is a principle that relates the vapor pressure of a solvent in a solution to its mole fraction. It states that the vapor pressure of the solution is equal to the product of the pure solvent vapor pressure and its mole fraction. This concept is fundamental in understanding how the addition of a solute reduces the vapor pressure of a solvent.

Colligative Properties

Colligative properties are characteristics of solutions that depend only on the number of solute particles present and not on their chemical nature. Vapor pressure lowering, along with boiling point elevation, freezing point depression, and osmotic pressure, are examples of colligative properties. These properties are important for comparing the effects of nonvolatile solutes and how they alter the physical behavior of a solution.

Mole Fraction

The mole fraction is a unitless measure that expresses the ratio of moles of one component to the total moles in a mixture. In the context of vapor pressure lowering, it is used to quantify the proportion of solvent present in the solution. Calculating the mole fraction of the solvent is crucial for applying Raoult's Law to determine the lowered vapor pressure.

Dissociation

Dissociation refers to the process by which an ionic compound breaks apart into its individual ions when dissolved in a solvent. This concept is significant in colligative property calculations, as a substance that dissociates releases more particles per formula unit compared to a non-dissociating compound, thereby exerting a larger effect on properties such as vapor pressure lowering.

Transcript

00:01 All right, so both a and b of this problem are going to be looking at the effect of adding a non -volatile solute to a solvent.

00:10 And so when we're looking at how the vapor pressure changes, that's going to use rawlts law, which tells us that the overall vapor pressure of the solution is going to be equal to the normal vapor pressure of the pure solvent times the mole fraction of the solvent.

00:26 And what that mole fraction does is give us a representation as a very much.

00:30 To how much of the solution is consisting of the solvent versus how much is due to the non -volatile salt ute.

00:37 All right, so let's go ahead and plug in what we know from scenario a, and then we'll repeat the process with scenario b.

00:44 So what they tell us is that our vapor pressure of water normally, which is our solvent, is going to be 71 .93 millimeters of mercury.

00:54 And so we have the first part of our equation.

00:57 And so what we need to find next is the mole fraction of the solvent.

01:02 And so when you're doing a mole fraction, all of your amounts of both your solvent and salute need to be in moles before you proceed.

01:09 So that's what we need to do next.

01:11 So they tell us that we have 10 grams of urea, which is ch4 and 2o, and so we're going to go ahead and convert that to moles using the molar mass.

01:19 Again, the molar mass is found by adding up the masses at the bottom of each square on the periodic table.

01:26 For urea, that happens to be 60 .07 grams per one mold.

01:30 We'll note to take 10 and divide by the 60 .07, and that tells us that we have 0 .166 moles of urea.

01:40 And this is going to be our solute.

01:43 It's what's been added to the water.

01:45 Now we'll repeat the process with water.

01:47 Here we have 150 grams of water and water smaller mass, again, by adding up two hydrogens and one oxygen from the period.

01:56 Table is 18 .02 gram for one mole.

01:59 We'll take 150 divide by 18 .02 since it's in the denominator, and that gives us that we have 8 .324 moles of water, and water in this case is going to be our saw vent.

02:14 So when we look at how the mole fraction for the solvent is actually written, the way it is is you take the moles of your solvent and you divide by the total.

02:26 Moles of solution.

02:28 And so what that ends up being for the total moles is you take the moles of solute plus the moles of your solvent and that gives you the total.

02:37 So for our example, our solvent is the water so that would go on top 8 .324 and then on the bottom to that moles of solvent 8 .324 we're going to be adding the moles of our solute urea .166.

02:55 And so when we go ahead and mark this out, 8 .324 divided by the sum of .166 plus 8 .34, that gets us a mole fraction of 0 .980.

03:10 And so what that means is that the majority of the solution consists of the solvent.

03:16 And so that's what we're going to be plugging in for the mole fraction of the solvent.

03:20 And then to our calculators, we'll plug in 71 .93 times the mole fraction of the solvent.

03:26 Fraction, 0 .980.

03:29 And that gets us a vapor pressure of 70 .5 millimeters of mercury for this first solution.

03:38 All right.

03:39 And there we go.

03:39 And we see that the vapor pressure has been slightly lowered from the original vapor pressure of just pure water.

03:46 And that matches our expectation with colligative properties.

03:49 So now what we're going to do is repeat the same process for a different solution.

03:57 This time instead of urea, we're going to be thinking about lithium chloride...

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