[big]

Assuming ideal behavior, which of the following aqueous solutions will have the lowers vapor pressure at 25°C?
a. 0.800 molal C₁₂H₂₂O₁₁
b. 6.00 g of urea (NH₂CONH₂) dissolved in 100. g of H₂O
c. 5.00 g of NaCl dissolved in 100. g of H₂O
d. χC₆H₁₂O₆ = 0.0200

so I might be wrong in my calculations, but otherwise I have that the mole fractions of water in each case is:
a. 0.200
b. 0.9823
c. 0.98482
d. 0.98

but then after that, how do you tell what the vapor pressure is?

[Borek]

What laws describing pressure over ideal solution do you know?

[opti384]

The vapor pressure of a solution will be lower than the pure solution and that magnitude depends on the molal concentration.

[big]

well there's raoult's law, which is that p=x_solv*P°p_solv. So is the answer A? but if I remember correctly, I think the answer was supposed to be D or something? does the van't hoff factor have anything to do with this problem?

[opti384]

does the van't hoff factor have anything to do with this problem?

Of course it does.

[big]

I figured out where I went wrong - I calculated the wrong mole fraction of water for answer A, but now I know how to get D, thanks 

[big]

does the van't hoff factor have anything to do with this problem?

Of course it does.

My apologies for reviving this, but what exactly does the van't hoff factor have to do with raoult's law? I don't see the van't hoff factor anywhere in raoult's law.

[Borek]

It is hidden in the way molar fraction is calculated.

[big]

Oh, that makes sense. So when calculating the molar fraction of NaCl in part c, do you just multiply by two because it dissociates into two ions?

[opti384]

Yes.