[ChaosSkies]

Hi

Have a question here from my general Chem 2 lab. The experiment involved recording the vapor pressure of 11 different combinations of 2-propanol and water. The data recorded was as follows.


mL 2-Prop   mL water   Ptotal (atm)             Patm = 1.006
 10                    0            1.036
  9                    1            1.039
  8                    2            1.036
  7                    3            1.033
  6                    4            1.036
  5                    5            1.031
  4                    6            1.023
  3                    7            1.026
  2                    8            1.018
  1                    9            1.015
  0                   10           1.013

From this we have to plot the mole fraction of the alcohol vs the Pvap. My question is I am unsure of how to calculate the mole fraction (Xa) of 2-propanol. We aren't given the mass of 2-propanol, so I am guessing we have to use Raoult's law (which makes sense considering we are graphing this data). I am not too experienced with Raoult's law though, and was wondering if anybody could help me start it off.

I know the equation:
Pa = XaPa°

I am unsure of what goes where though. I know the Ptotal goes in the Pa, and we are solving for Xa, but what about the Pa°; how do you figure it out?

Any help is greatly appreciated.

 

[Yggdrasil]

You can get the masses of the 2-propanol and water from their volumes if you know the densities of the two substances.

[ChaosSkies]

I did think about using the density to try and calculate the mass, but we are supposed to be able to calculate the mole fraction using only that above data. In other words, we're not supposed to look up the density of either substance; the 2-propanol or the water.

This is a big reason why im struggling with this particular problem, I don't know how to calculate the mole fraction with only the data provided.

[Yggdrasil]

It makes a lot more sense to me if you were to calculate the mole fractions from the density, but if you want to calculate the mole fractions from your data then you can use this equation

P_total = P_a + P_b

Where P_a = P_a^oX_a and P_b = P_b^oX_b.

Basically, from Raoult's law you can get the partial pressure of each gas and the sum of the partial pressures of both gasses will give you the total vapor pressure over the solution.  Note that the data tell you the partial pressures of pure water and pure 2-propanol.

[ChaosSkies]

I agree with you on the density, our chem lab professor just wants us to do it like this, thats all.

OK, so i get this part

Ptotal = pA + pB = pA* xA + pB* (1- xA)

I'm still a little confused though. is pA* and pB* both the Pvap of the pure substances (the first trial for 2-propanol and the last trial for water). If this is correct, then can you just plug these into the formula, and solve for the mol fraction (xA) ?

Sorry if im asking fairly basic questions, I just want to totally understand what im doing.

Thanks again!!!!

[Yggdrasil]

Yes.  pA* and pB* are the vapor pressures of the pure substances, and you do have all the data you need to solve for the mole fractions.

What would be an interesting thing to do is to calculate the mole fractions from the density and from the vapor pressure data.  Then you can compare how the theory (Raoult's Law) matches with reality (the actual number of moles you used to make the solutions).

[ChaosSkies]

Yes, comparing the mole fraction using density with the mole fraction using the vapor pressure would be pretty interesting. Its always cool to see how close theoretical values compare to the true measured values.

Thanks again for all your *delete me*!!