mole fraction (gen chem question)

[patricklin27]

http://img850.imageshack.us/img850/2286/6jg.png

the answer is 0.5 but i got 0.3.

shouldn't the mole fraction be 535/(1070+535). Isn't the total pressure both pure water and glycol, and not just water?

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[game33]

I see how they did it. I also see how you did it. I would have done the same thing as you, but that's why people on SDN say don't use Qvault Chems or anything but Bio because it has a lot of mistakes. I don't see why they solved it they way they did.

 

[patricklin27]

yeah some of the question seem suspect on there. oh well. anybody else got an opinion on this question?

 

[Illfavor]

Mole fraction = moles solvent/(moles solvent + solute)
Raoult's Law: Pressure of solution= mole fraction solvent X pure pressure solvent.
535= X(1070), X= .5

If the mole fraction of the solvent is .5, then the mole fraction of the solute must be, by definition, .5.

10 mole H20/20 mole total = .5. This means we have equal amounts solute/solvent.

 

[patricklin27]

Mole fraction = moles solvent/(moles solvent + solute)
Raoult's Law: Pressure of solution= mole fraction solvent X pure pressure solvent.
535= X(1070), X= .5

If the mole fraction of the solvent is .5, then the mole fraction of the solute must be, by definition, .5.

10 mole H20/20 mole total = .5. This means we have equal amounts solute/solvent.

Raoult's law is vapor pressure of solution= mole fraction of component in mixture x partial pressur of component in mixture...... not solvent.


Therefore, the mole fraction of the component in mixture would be 535/(1070+35) since you have to account for the total pressure of both solute (ethlyene) and solvent (water).

 

[Illfavor]

Raoult's law is vapor pressure of solution= mole fraction of component in mixture x partial pressur of component in mixture...... not solvent.


Therefore, the mole fraction of the component in mixture would be 535/(1070+35) since you have to account for the total pressure of both solute (ethlyene) and solvent (water).

The way you're setting it up is incorrect. The bolded part makes no sense in this problem. You don't care about the pressure of ethylene glycol, only the mole fraction and the pure pressure of water, things given to us. We are solving for ethylene glycol by solving for water (given the data we have) and inferring the answer from there.

There are many ways to say the law....but let's do it your way. If solute/solvent don't matter, then just take any component. In this case, your "component" in the mixture is water.

VP of solution = mole fraction H2O * pure vapor pressure of H2O.
535 = X * 1070, X= 5 = mole fraction of water in this solution.

Therefore, the mole fraction of the other component must also be .5 ( since it's effectively a percentage and must add up to 1).

 

[Xo1991]

Here is why you are wrong and qvault is right:

I believe you are getting slightly mixed up with vapor pressure and partial pressure. Vapor pressure usually refers to solutions of water, etc. Partial pressure is with gases, where the sum of partial pressures of the constituents is equal to the total pressure of the gases.

When doing vapor pressure problems like this, you only really care about the vapor pressure of the pure solvent

Raoult's law refers to vapor pressure. Dalton's law refers to partial pressure of gases.

(Also notice that qvault lists this is a 'Colligative Properties' question, not 'Gases')