# Vapor Pressure of saturated/unsaturated solution

• Joshua Kenny
In summary, the two solutions are saturated, 1 has a higher boiling point, and one has a smaller PH2O value.
Joshua Kenny

## Homework Statement

:

As the picture shows, there are two NaClaq solutions. One is saturated while the other is not and both of them are at the same temperature. There are three questions asked:
a) Above which solution is the vapor pressure of water, PH2O, greater? Explain.
b) Above one of these solutions, the vapor pressure of water, PH2O, remains constant, even as water evaporates from solution. Which solution is this? Explain.
c) Which of these solutions has the higher boiling point? Explain.

2. Homework Equations

As Raoult's law states: Psolution=Psolvent*Xsolvent
where Xsolvent is the mole fraction of the solvent in the solution. In this case, Xsolvent is XH2O.[/B]

## The Attempt at a Solution

**For question a), solution (2) has a greater PH2O value because the amount of solute (in this case NaCl) is less than that of solution (1) therefore Xsolvent of solution (2) is greater than that of solution (1).

**For question c), solution (1) has a smaller PH2O value and therefore the boiling point of solution (1) would be higher than that of solution (2).[/B]

I'm left confused by question b) because I cannot prove how PH2O could remain constant. I would be glad if someone could explain the logic of this question. Please check as well if the previous answers/explanations for question a) and c) are appropriate or not.

#### Attachments

• nacl.png
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I've made a mistake for question a) & c)

For question a), solution (2) has a greater PH2O value because the amount of solute (in this case NaCl) is less than that of solution (1) therefore Xsolvent of solution (2) is greater than that of solution (1).

For question c), solution (1) has a smaller PH2Ovalue and therefore the boiling point of solution (1) would be higher than that of solution (2).

What happens to the solutions as water evaporates? Describe both cases.

As water evaporates, the amount of solvent (water) in both solutions decreases. For solution (2) the mole fraction of water should decrease in value. If I'm not mistaken, for solution (1) (which is saturated), as the water evaporates, some of the salt solidifies as well in proportion. So, the mole fraction of solvent of solution (1) remains constant.

Is this correct?

Very good.

So the answer is definitely 2,1,1?

Joshua Kenny said:
One is saturated while the other is not
"Not saturated, saturated, saturated."

Thanks a bunch!

## 1. What is vapor pressure?

Vapor pressure is the pressure exerted by the gaseous form of a substance in equilibrium with its liquid or solid form at a given temperature. It is a measure of the tendency of a substance to escape into the gas phase.

## 2. What is the difference between saturated and unsaturated solutions?

A saturated solution is one in which the maximum amount of solute has been dissolved in a given amount of solvent at a given temperature. An unsaturated solution has not reached its maximum solubility and can dissolve more solute.

## 3. How does temperature affect the vapor pressure of a solution?

In general, as temperature increases, the vapor pressure of a solution also increases. This is because at higher temperatures, more molecules have enough energy to escape into the gas phase, resulting in a higher vapor pressure.

## 4. How does the presence of a solute affect the vapor pressure of a solution?

The presence of a solute in a solution lowers the vapor pressure compared to the pure solvent. This is because the solute molecules disrupt the surface tension of the solvent, making it more difficult for solvent molecules to escape into the gas phase.

## 5. How is vapor pressure measured experimentally?

Vapor pressure can be measured using a variety of methods, such as the static method, dynamic method, or Knudsen effusion method. These methods involve creating a known volume of vapor above the solution and measuring its pressure at a given temperature.

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