[HiThisIsAName]

Okay. So. The vapour pressure above a glucose-water solution at 25 degrees is 23.8 torr. The vapour pressure of water at 25 degrees is 30.5 torr. What is the mole fraction of glucose in the solution?

A) .77

B) .22 (correct answer)

C) .44

D) .55

I have the workings and the answer, but I don't understand them. Any help would be appreciated. If possible, I'd like a method of working this problem out - I.e. a step-by-step guide sort of thing, since I'm really lost with this one.

P.S. Sorry if this is the wrong section. I tried to pick the best one.

[LQ43]

Do you know Raoult's Law?

show the workings and at which point you don't understand them

[HiThisIsAName]

P_solution= (X_solvent) P^o_solute

X_solution= (1-X_solute) P^o_solvent

Therefore, 23.8= (1-X_solute) 30.5

(1-X_solute)= 23.5/30.5

                              = 0.7803
                           
                              = -0.7803 - 1

P= -0.2196X_solute

So, P= 0.2196

None of that makes any sense to me at all. Yes, I have heard of Raoult's Law, but I'm not sure how to apply it in this case.  Also, I don't know if X is a widely used symbol for mole fraction, but that's what it represents. I'm a Biologist! I should be doing something involving things I can actually see.   

[LQ43]

P_solution= (X_solvent) P^o_solute

should be P^o_solvent

1. P_solution = X_solventP^o

Find X_solvent

2. Key: 1 = X_solvent + X_solute

Find X_solute

here's a visual below and an animation does this help?

http://www.chem.iastate.edu/group/Greenbowe/sections/projectfolder/flashfiles/propOfSoln/vp3.html

[HiThisIsAName]

Thanks for that! It helped a lot.

Now to move on to Physics...I wish I was back at high school.