Vapor Pressure of saturated/unsaturated solution

[Joshua Kenny]

Homework Statement

:

As the picture shows, there are two NaCl_aq 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, P_H2O, greater? Explain.
b) Above one of these solutions, the vapor pressure of water, P_H2O, 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: P_solution=P_solvent*X_solvent
where X_solvent is the mole fraction of the solvent in the solution. In this case, X_solvent is X_H2O.[/B]

The Attempt at a Solution

**For question a), solution (2) has a greater P_H2O value because the amount of solute (in this case NaCl) is less than that of solution (1) therefore X_solvent of solution (2) is greater than that of solution (1).

**For question c), solution (1) has a smaller P_H2O 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 P_H2O 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.

 

[Joshua Kenny]

I've made a mistake for question a) & c)


For question a), solution (2) has a greater P_H2O value because the amount of solute (in this case NaCl) is less than that of solution (1) therefore X_solvent of solution (2) is greater than that of solution (1).

For question c), solution (1) has a smaller P_H2Ovalue and therefore the boiling point of solution (1) would be higher than that of solution (2).

 

[Bystander]

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

 

[Joshua Kenny]

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?

 

[Bystander]

Very good.

 

[Joshua Kenny]

So the answer is definitely 2,1,1?

 

[Bystander]

One is saturated while the other is not

"Not saturated, saturated, saturated."

 

[Joshua Kenny]

Thanks a bunch!