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TBR: Vapor Pressure and Partial Pressure

Discussion in 'MCAT Study Question Q&A' started by justadream, Sep 5, 2014.

  1. justadream

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    Are vapor pressure and partial pressure different in this scenario?

    TBR GenChem Book II page 105 #42

    You start off with a mixture of 2-propanoal with acetophenone.

    "Addition of another 60g of 2-propanal to the original mixture wold affect the vapor pressure of aceteophenone in what way?

    Answer: It would decrease the vapor pressure of aceteophenone


    TBR's explanation says that because the mole fraction of acetophenone decreases, the vapor pressure of acetophenone decreases.

    What about the partial pressure of acetophenone? Is that constant or does that decrease as well?
     
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  3. DrDreams

    DrDreams Captain of the U.S.S. Enterprise
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    (in no way is this meant to sound condescending, it is just the way I think of it) Yes, because think about it as you looking at a ball pit from above. The balls like to be popped up, but when there are equal amounts of blue and green balls, they will have an equal vapor pressure. However, when I have two times the amount of blue balls, the chance of a green ball getting above the ball pit decreases. I know it sounds dumb, but thats just the way I think of it and it works for me.
     
  4. justadream

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    @DrDreams

    Right, I get that the fraction of the thing that was not added will decrease.

    But it still doesn't make sense to me why the contribution of anything unchanged doesn't stay the same.

    For example, let's say I had a gas made up of Gas X and Gas Y.

    Total Pressure = (Partial Pressure of Gas X) + (Partial Pressure of Gas Y)

    If I add more of Gas X, the total pressure will increase. But doesn't the partial pressure of Gas Y stay the same?


    ================
    Sudden realization while writing what I wrote above:

    I think I can reconcile this situation by thinking of vapor pressure as the SUM of the partial pressures.

    So if you change the mole fractions, then the vapor pressure can change. However, the individual partial pressures remain the same.
     
  5. DrDreams

    DrDreams Captain of the U.S.S. Enterprise
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    So if you change the mole fraction, you've added more of that gas, hence its like suppressing the other gas. That's is how I think of it. Majority rules (in vapor pressure)
     
  6. justadream

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    @DrDreams

    Right. Would you agree that while the vapor pressure changes in this situation (adding more of one gas), the partial pressure of the gas (the one that is not added) is unaffected?
     
  7. DrDreams

    DrDreams Captain of the U.S.S. Enterprise
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    No I disagree. I believe the that the partial pressure of the gas that has not been added will decrease. And the partial pressure of the gas that has been added will increase.

    BTW reread your second post. The reason it's vapor pressure decreases is because it's suppressed. It is basically overwhelmed by the majority and can't make it out into the vapor pressure as easily as it once used to.
     
    #6 DrDreams, Sep 5, 2014
    Last edited: Sep 5, 2014
  8. justadream

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    @DrDreams

    Wiki defines partial pressure as:

    "In a mixture of gases, each gas has a partial pressure which is the hypothetical pressure of that gas if it alone occupied the volume of the mixture at the same temperature."

    http://en.wikipedia.org/wiki/Partial_pressure

    So if you don't add any of Gas Y (in a mixture of Gas X and Gas Y) but you add Gas X instead, doesn't the pressure of Gas Y (if it existed by itself) stay the same?
     
  9. DrDreams

    DrDreams Captain of the U.S.S. Enterprise
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    No. Because the addition of it will drive gas x to concert to the liquid state and along with it gas x. The vapor pressure will be the same but the ratio distribution of gases is altered.
     
  10. justadream

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    Are you saying the total vapor pressure is the same or the VP of an individual gas?
     
  11. DrDreams

    DrDreams Captain of the U.S.S. Enterprise
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    The total will remain the same. I remember this dilemma from tbr Gen Chem as well. The way I imagined was that this is done inside a flask. The thing is the total vapor pressure will remain the same because after the addition of gas x gas y is unable to leave the liquid state as easily because surrounding it are more gas x molecules. Therefore it's mol fraction has gone down hence it's vapor pressure as well. But the vapor pressure that is lost by gas y is compensated for by the vapor pressure of gas x increasing.
     
  12. DrDreams

    DrDreams Captain of the U.S.S. Enterprise
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    But thinking about it if the mol fraction of gas x has increased and if the total vapor pressure has increased then the partial pressure of gas x has increased and if the partial pressure of gas y remains the same the actual partial pressure of it relative to has x (as a ratio) is less right?
     
  13. justadream

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    @DrDreams

    Wait I'm confused about total vapor pressure being the same. Don't these type of graphs (see below) show that when you change the fraction of one gas, the total vapor pressure changes?

    Also, I'm confused about your question because I thought you just said that the total vapor pressure is constant (in your question you're saying it increased)?


    [​IMG]
     
  14. DrDreams

    DrDreams Captain of the U.S.S. Enterprise
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    I apologize, I may have been wrong about the total pressure, because I was going based off a similar question I had, but it had a picture of a flask in it, and that is what I was thinking of.. I didn't have access to my TBR books but I just got them and looked it up. BUT I think my picture below will explain it in terms of even if the mol fraction of gas X is the same, then why is the partial pressure of that gas lower? I think its relative to the other gas man. Thats the only thing I can think of. So I think you got it, I had just confused you about the vapor pressure remaining the same.

    "Sudden realization while writing what I wrote above:

    I think I can reconcile this situation by thinking of vapor pressure as the SUM of the partial pressures.

    So if you change the mole fractions, then the vapor pressure can change. However, the individual partial pressures remain the same."

    IS correct and that is the BEST way to think of it. Its relative. I have the picture on page 86 GCII is the same pic. but you are right. Vapor pressure is the SUM of partial pressures.

    Let me know if my picture helps. Your right, if you add the Gas Y its mole fraction has definitely increased, and hence its vapor pressure has increased. The total vapor pressure is now 2 compared to 0.5 before go Gas Y. BUT the vapor pressure of gas X is the same, but relative to the other gas, the vapor pressure has decreased.

    [​IMG]
     
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  15. justadream

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    @DrDreams

    Lol after thinking about this for another 30 minutes, I'm not sure if what I wrote is correct. I think vapor pressure is only dependent on the mole FRACTIONS - and has NOTHING to do with the total amount of gas in it.

    Here's my though experiment:

    Let's say you have an equal-molar mixture Gas X and Gas Y. Gas X is more volatile than gas Y.

    1) Now if you add Gas X, the fraction of Gas X increases and the vapor pressure of both Gas X and the total vapor pressure (which approaches the vapor pressure of pure Gas X) increases.

    2) If you add Gas Y, the fraction of Gas Y increases and the VP of gas Y increases. Here, would the total vapor pressure decrease (since you have more of Gas Y - the less volatile gas)?



    If you think that the total vapor pressure would decrease in scenario 2, how can you reconcile that with the fact that whenever you add a gas, the total pressure intuitively seems like it should increase? For example, if I added 10000 moles of Gas Y, I'd expect the pressure to increase overall.

    Thus, I'm not sure if I can establish a relationship between total pressure and total vapor pressure.
     
  16. DrDreams

    DrDreams Captain of the U.S.S. Enterprise
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    I really don't know what to say. I mean I have been thinking of the vapor pressure as constant (even though it may not be correct) but Ive been getting the questions right. So I am not sure what to say. But what you said makes sense to me, both 1 and 2. Its just relative. I don't know how to explain it though.
     
  17. justadream

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    @DrDreams

    The more I think about it, the more confused I get.

    With my luck, my real MCAT is gonna require knowing this distinction and I will get screwed....
     
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  18. DrDreams

    DrDreams Captain of the U.S.S. Enterprise
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    There is never ever knowing everything for the MCAT. I was scared about that too and had been content reviewing like a madman. However, I just finally started taking practice exams and thank God, I was surprised at how much I knew vs how much I thought I knew. Just attack it. and use the AAMC exams to judge how much you ACTUALLY know in comparison to what you think.
     

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