# Homework Help: Vapor pressure saturation

1. Nov 15, 2008

### johnj7

1. The problem statement, all variables and given/known data
"Suppose you were to collect 1.0 L air by passing it slowly thorugh water at 20C and into a container. estimate the mass of water vapor in the collect air, assuming that the air is saturated."

I don't know where to even begin. at first i thought it had something to do with henry's law, but that didn't lead me anywhere

2. Relevant equations
Henrys Law
S=kH X P
3. The attempt at a solution
I have no idea, I read the chapter on physical equilibria throughly, but I can't answer this question. All I need is the conceptual understanding as to how I would solve this problem. The only way I thought of doing this is
1. find the solubility of air in water (henry's constant given)
2. plug in the vapor pressure of water at 20C for P
3. Obtain solubility
4. then I have the solubility of air in water
5. that didn't get me anywhere

I couldn't think of anything else, conceptual guidance would be appreciated
the answer is I don't know where to even begin. at first i thought it had something to do with henry's law, but that didn't lead me anywhere

thanks !

2. Nov 15, 2008

### Bystander

Apply Dalton's law of partial pressures to the vapor phase.

3. Nov 15, 2008

### johnj7

I thought about it, but still can't figure out how to do it... it shouldn't be that hard, its the first problem in the chapter!!

so daltons law says that the total pressure is equal to the mole fractions of each species multiplied by the total pressure
ie
Ptot = X1P + X2P

I have that the vapor pressure of water at 20C is 17.54
what can I do now?

4. Nov 15, 2008

### Bystander

You've got volume, you've got pressure, you've got temperature. What do you know about ideal gases?

5. Nov 15, 2008

### johnj7

wow simple
i keep forgetting to always think about the ideal gas law
thank you so much