Partial Pressure Question

  1. Hello, first post here.
    Here is a hypothetical partial pressure of gas question

    Imagine you have a two component system:
    Component 1 is water
    Component 2 is an imaginary substance that is immiscible with water, but has same boiling temp / pressure

    You put the two components in a pressurized cylinder with equal molar volume distribution, and set the pressure to 1atm and 99°C (of course we know pure water will be liquid at this stage). Partial pressure law implies that water (or steam) will be exerting .5 atm, as would component 2 (.5 atm and 99°C will yield a vapor). Will there be liquid water or steam inside the cylinder?

    Thanks for the help!
  2. jcsd
  3. Suppose you had pure liquid water in a container (with no imaginary immiscible substance present), and you held the pressure at 0.5 atm (absolute) while raising the temperature to 99 C. What would the state of the contents of the container?

  4. You would be in a vapor phase
  5. To be more precise, you would have superheated steam at 0.5 atm and 99 C. There would be no liquid water present. Now you do the same problem for the pure immiscible substance at 0.5 atm absolute and 99 C. What would be the state of the immiscible substance?
  6. Oh yes you are correct. Imagine the immiscible substance has the same thermodynamic properties. I am just wondering if water should be treated as if it is under 1atm conditions or .5atm due to partial pressure. Meaning if the water in the 1atm cylinder would have vaporized or remained as a liquid, when there is another substance in the cylinder taking up have the volume and exerting half the pressure.

    Sorry if I am explaining the question poorly. I am vaporizing hydrocarbons and water and am wondering if I should take into account partial pressures or not.
  7. Yes. I understand what you are trying to figure out. I'm just trying to help you figure it out by leading you to the solution gradually. So, please answer my question in my previous post about the pure immiscible substance.

  8. The pure immiscible substance has the same thermodynamic properties as the water. So at .5atm and 99°C, it is a gas. At 1atm and 99°C, it is liquid.
  9. Excellent. Now you have a cylinder that has equal molar amounts of water and immiscible substance at 1 atm, such that the partial pressure of each is 0.5 atm. The temperature is 99C.

    Assuming that they are both superheated vapors in the gas phase, what is the saturation temperature (dew point) for each of these substances corresponding to a saturation vapor pressure is 0.5 atm?

  10. The saturation temperature would be 82°C at .5atm for both substances
  11. So, if nothing would condense out, you have your answer to your original question.
  12. Okay, so just to recap, both fluids will be in gas phase since both should be in gas phase at .5 atm at 99C, even though the cylinder is set to 1atm, due to partial pressure of .5atm for each substance. Is this correct?
    Thanks for your help in this.
  13. Yes. You pretty much doped this out for yourself.

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