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Homework Help: Gas Laws Review Help?

  1. Jan 11, 2012 #1
    1. The problem statement, all variables and given/known data
    Hydrogen gas is collected over water at 25°C. The atmospheric pressure in the room is 745 mm Hg and the volume of gas in the container is 43.5mL. The vapor pressure of water at 25°C is 23.8 mm Hg. Determine the volume the gas would occupy at STP.

    2. Relevant equations
    standard pressure=760 mm Hg or 1 atm
    standard temperature= 273 Kelvin

    3. The attempt at a solution
    Well for this one I wasn't really sure if they just gave me excess information to throw me off. My first response was to say that the volume would just be 43.5mL. None of the equations provided above worked with just the information given. Is this really as easy at it seems?
  2. jcsd
  3. Jan 12, 2012 #2


    User Avatar

    Staff: Mentor

    No, it is not that easy, and there is no excess information.

    First of all, you know the volume at 745 mmHg and 25°C, these are not STP conditions, so the answer can't be 43.5mL.

    Then, some of the volume is taken by the water vapor, and you are asked about the volume of just hydrogen - so you have to take 23.8 mmHg into account. What is the pressure of the dry hydrogen in the collected sample?
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