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Homework Help: Determing vapour pressure experimentally - how?

  1. Oct 8, 2011 #1
    1. The problem statement, all variables and given/known data

    I am supposed to determine the vapour pressure experimentally of a series of prepared solutions, which are a mixture of 2 different substances in different proportions.
    This involved attempting to create a "vacuum", placing the liquid inside and then measuring the change in pressure.

    I was then supposed to compare those values to the "ideal" values I would get, which were calculating using Raoult's law. My problem is, I know how to figure out the change in pressure after I added the liquid, but how does this change in pressure relate to the vapour pressure?
    (The room pressure was about 770 torr, and after the "vacuum" we had a change of roughly 330/340 torr between room and experimental pressure)

    3. The attempt at a solution

    I really haven't tried much because I don't know where to begin. I calculated the difference between pressure before and after the addition of the liquid, but I don't know how to deduce the vapour pressure from that. The vapour pressure isn't simply the change in pressure, right? Because the values we obtained were in average about 5 or 6 times lower than what was expected. Also, it makes sense to me that the change in pressure we registered would be dependent on the pressure that the liquid was experiencing inside. The higher the pressure, the less it would evaporate.

    Thanks for whatever help you can give. If you need more info, please ask. I didn't provide everything because I feel I'm just stuck in that particular step, and after I understand that I'll be able to finish it.
    Last edited: Oct 8, 2011
  2. jcsd
  3. Oct 8, 2011 #2
    So are you supposed to design the experiment to determine the pressure change? Or are you given some experimental data that you're supposed to manipulate to find an actual pressure change & then compare to the theoretical change? I might just be reading at a lower-than-usual level, but I'm kind of confused about the answer to this question, which makes it problematic in trying to proffer a solution path.
  4. Oct 8, 2011 #3
    I am designing the experiment and determining the pressure change, yes.

    I'm a bit confused about it aswell. I have to get all the data myself. The only data I'm given is some constants which I then use to calculate the theoretical vapour pressure of a mixture of those substances, using Raoult's Law.

    After that, I prepare those solutions and depressurize the container. After that, i check the initial pressures difference (between inside and outside) which was normally about 330 or 340 torr and then I register the pressure after adding the liquid and giving it enough time to reach equilibrium.

    With this data, I'm meant to be able to calculate the experimental/real vapour pressure of the mixture of substances I added to the container, but I just don't understand how.
    How can I relate the difference in pressure to the actual vapour pressure?
  5. Oct 8, 2011 #4
    As far as designing the experiment, you could set up a pressure gauge consisting of an end-piece of known area A and a spring with a known spring constant k and begins (when pressure is 0) in an equilibrium position. The force acting on the pressure gauge will be the pressure in the apparatus * the area of the end-piece, which will also equal -kx where x is the displacement of the end-piece due to the force exerted on it. (Because the external force from the fluid pressure will place the spring system in a new equilibrium position, the force acting on the end-piece of the gauge will have equal magnitude to the restoring force exerted by the spring itself.) If you have a way to measure the displacement, you can solve the equation for F, then substitute F in to solve for your pressure.

    Beyond that, I don't know what I can do to help since it's been probably about 3 years since I've taken a class that had anything to do with fluid pressure. :(
  6. Oct 8, 2011 #5
    I think I misuderstood your question, sorry. The experiment has already been done. Now I'm trying to take some conclusions from the data I already have.

    My only problem is the relation between the change in pressure and the actual vapour pressure. But thanks for trying anyway
  7. Oct 8, 2011 #6
    Oh. Well, the best I can think is to manipulate the equation PV = nRT using the values you've got or can readily derive. I wish I could get more in-depth, but the wife is hollering at me to go with her to watch some football.

    Oh, also, shouldn't your vapor pressure just be a function of the measured final and initial pressures?
  8. Oct 8, 2011 #7
    Yes I think it has to be a function of both, but I have no idea on how they are related, really. I mean, I know the change in pressure caused by the gas phase that originated from the addition of the liquid, but then I get stuck. It's probably also because I don't think I fully grasp the concept of vapour pressure. As in, what happens when a liquid with a vapor pressure of, say, 300 torr is put in a container with a pressure of 100 torr? How many torrs is the pressure raised until it reaches an equilibrium? What if it was 500 torrs instead of 100? I think if I was able to calculate that, I'd be able to solve this.

    Also about the PV=nRT, I doubt that's the way.
    Last edited: Oct 8, 2011
  9. Oct 8, 2011 #8
    If the liquid doesn't vaporize then it just changes the volume your gas has to move around in,doesn't it?
  10. Oct 8, 2011 #9
    But that's the deal, it does vaporize. Also the container was big enough to consider the volume of liquid added as irrelevant as far a changes in pressure go
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