[AzMa1]

Because I was having some difficulty solving a problem on vapor pressure (which i think my teacher mistakenly gave us as the chapter we are on makes no mention of vapor pressure, just partial pressure), I decided to see if the solutions to the problems were online. Luckily, they were but I'm still having some difficulty understanding.
For example:
" At 20oC the vapor pressure of benzene is 75 mm Hg, and the vapor pressure of toluene is 22 mm Hg. Solutions in both parts of this question are to be considered ideal.
(a)   A solution is prepared from 1.0 mole of biphenyl, a nonvolatile solute, and 49.0 moles of benzene.  Calculate the vapor pressure of the solution at 20oC.
(b)   A second solution is prepared from 3.0 moles of toluene and 1.0 mole of benzene.  Determine the vapor pressure of this solution and the mole fraction of benzene in the vapor.
Answer:
(a)   PC6H6 = XPoC6H6  = (49/50)(75) = 73.5 mm Hg
(b)   Ptot = XPotol. + XPobenz. = (3/4)(22) + (1/4)(75) = 16.5 + 18.75 = 35.3 mm Hg
mole fraction in vapor = 18.75/35.3 = 0.531"

All I have learned so far is that P_a=X_aP_total, Dalton's law, and that P_total=n_total(RT/V). So I don't understand the whole mole fraction being multiplied by vapor pressure thing. Also, shouldn't the mole fraction of benzene in the vapor just be be 1/4=0.25 not 0.531?

Also, in another question:
" A 6.19 gram sample of PCl5 is placed in an evacuated 2.00 liter flask and is completely vaporized at 252oC.
(a)   Calculate the pressure ion the flask if no chemical reaction were to occur.
(b)   Actually at 252oC the PCl5 is partially dissociated according to the following equation: PCl5(g) ® PCl3(g) + Cl2(g).
        The observed pressure is found to be 1.00 atmosphere.  In view of this observation, calculate the partial pressure of PCl5 and PCl3 in the flask at 252oC.
Answer
(a)   6.19 g PCl5 / 208.22 g/mol = 0.0297 mol PCl5
            P = nRT/V = (0.0297)(0.0821)(525)/(2.00) = 0.640 atm
(b)   PPCl3 = PCl2 = x; PPCl5 = (0.640 - x)
        PT = 1.00 atm = (0.640 - X) + X + X
        X= 0.360 atm = PPCl3 = PCl2
        PPCl5 = (0.640 - 0.360) atm = 0.290 atm = 220 mm"

Why is P_PCl3 considered equal to P_Cl2?

Thanks for any responses.

[Borek]

So I don't understand the whole mole fraction being multiplied by vapor pressure thing.

This is just Dalton's law - which you already know..

Also, shouldn't the mole fraction of benzene in the vapor just be be 1/4=0.25 not 0.531?

No. You confuse solution composition with vapor composition, they are not identical.

Why is P_PCl3 considered equal to P_Cl2?

Reaction stoichiometry.

[AzMa1]

What is vapor pressure in respect to partial pressure?
For some reason I'm getting caught up on this.

Also, why are pressures used for a mole fraction in the vapor?