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Homework Help: How would water vapor contamination affect PV=nRT

  1. Feb 21, 2013 #1
    1. The problem statement, all variables and given/known data

    Doing a lab here, but were kind of stuck. This lab is meant to experiment the formula PV=nRT, and the set up is a canister of CO^2 with a fixed volume that has a pressure sensor connected to it reading in pascals. We heat up the CO^2 by placing the canister in a beaker of boiling water and then slowly cool the water and take measurements until it reaches 0 degrees celsius. These results were placed in an excel spreadsheet to make a graph of P vs V, and this is used to extrapolate the value for absolute zero in celsius. Our answer was -302.9 which is obviously off from -273.15.
    Suppose the trapped air were contaminated with water vapor. How would this affect the P vs V graph? Do you see any such effect?

    2. Relevant equations


    3. The attempt at a solution

    We think the answer is yes and this is our source of the discrepancy, but we aren't sure. If the water vapor did contaminate the air, would that increase n in moles in the formula PV=nRT to make our graph offset too low like it did?
  2. jcsd
  3. Feb 23, 2013 #2


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    cool experiment. Although, it sounds kind of dangerous to be heating up pressurised canisters?! I guess the teacher knew beforehand that they could take the temperature change which you guys were subjecting them to?

    Anyway, yeah, water vapour contamination sounds like a possible reason. It is not just because this would change n though. Think about the equation. Do you trust that the equation is true even if there is water vapour contamination. A hint is in the name of the equation.

    Edit: ah, whoops! For some reason I got confused between water droplet contamination and water vapour contamination. I was writing water vapour, while thinking water droplets... Uh, still even with water vapour, it might affect the results. Yeah, your explanation of the water vapour causing a change in n from what you would expect sounds reasonable.
    Last edited: Feb 23, 2013
  4. Feb 23, 2013 #3


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    Actually, If we assume that water vapour is still an ideal gas, then would this change your prediction of absolute zero?

    Edit: think about the equation, and if n was different, for example, n+m, then would your prediction be different?
  5. Feb 23, 2013 #4
    This description is very confusing. Before we can help you, you need to help us out.

    1. You have a canister with CO2 in it. Where did the air come from?
    2. The volume of the canister is fixed. Did you plot P vs V, or P vs T?
    3. By what physical mechanism would water vapor get into the closed canister?
    4. What was the starting pressure of the gas in the canister? At the range of pressures and temperatures involved, is CO2 expected to behave like an ideal gas?
    5. Is the measured pressure absolute pressure or gage pressure (relative to the room pressure)?
    6. How do you know that the canister was thermally equilibrated with the water bath at each temperature?
    7. Was the canister fully submerged, or was part of it sticking out of the water?
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