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Homework Help: Vapor Pressure

  1. Nov 17, 2013 #1


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    1. The problem statement, all variables and given/known data

    A 425-mL sample of hydrogen is collected above water at 35°C and 763 torr. Find the volume of the
    hydrogen sample when the temperature falls to 23°C, assuming the barometric pressure does not change. (vapor pressures of water : at 35°C, 42.2 torr ; at 23°C, 21.1 torr)

    2. Relevant equations

    Okay, we're looking a vapor pressure problem. Therefore the partial pressure of the gas above water is found by subtracting the vapor pressure from the total, barometric (?) pressure. I'm assuming that's what barometric pressure refers to in this problem - that the total pressure of 763 torr is the barometric pressure, and it doesn't change.

    We're also looking at a problem that has three variables changing - the pressure (note the drop in vapor pressure) - the temperature (note the change in temperature stated in the problem) and the volume (the variable we're solving for). Therefore we should use PV/T = PV/T.

    3. The attempt at a solution

    Okay. Initial scenario:

    Pressure of hydrogen gas is 763 torr - 42.2 torr.
    Temperature = 35 + 273 K
    Volume = 0.425 L

    Final scenario:

    Pressure: 763 minus 21.1 torr. (Barometric (total?) pressure does not change according to the problem.)
    Temperature = 23 + 273 K
    Volume = variable we're solving for.

    Now, PV/T = PV/T. That's pretty simple and I just plugged it all into my graphing calculator.

    V = 0.396 liters.


    1) I know I got the right answer. Is my line of reasoning and process right?

    2) What exactly is barometric pressure? I think it's referring to total pressure within the closed system. This makes finding the partial pressure of hydrogen gas a lot easier because we can just do 763 (barometric pressure) minus the partial pressure of water vapor at whatever temperature instead of having to account for both changing barometric and vapor pressures at different temperatures.
  2. jcsd
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