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Homework Help: I need help on a chemistry problem PLEASEEE

  1. Feb 24, 2006 #1
    Ok, I need to calculate the vapor pressure of water.

    All the information I am given is that the temp is 43.0 C, and I know the total pressure of the system is 758.3 torr.

    Is there anyway I can calculate this using the Pwater = Ptotal - Pair equation? PLEASE HELP!! Thanks guys

  2. jcsd
  3. Feb 25, 2006 #2
    Is it something related to the equation for ideal gas? but in pV = nRT, the "n" has changed cos in 43.0 C water is liq but vapor is gas. Sorry, i've got no idea about it since I'm doing biology...
  4. Feb 25, 2006 #3


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    first things first, state the full problem.
  5. Feb 25, 2006 #4
    Full Problem

    um.. that is the full problem. PV=NRT gives me the total pressure. I just need to find the vapor pressure of water.

    This is what the problem says.
    Consider the following data:
    Temp = 43.0C Volume = 6.63ml
    Barometric Pressure = 758.3 torr

    a. Find the moles of air trapped in the cylinder

    Ok, I can do this part by solving for n, in the ideal gas law. This is not a problem... the second part is the problem.

    b. Calculate the vapor pressure of water, Pwater, at 43.0C

    THIS IS THE FULL PROBLEM. Now I would really appreciate it if you can help. Thanks Guys..

  6. Feb 26, 2006 #5
    Assume that the total pressure is 760 torr and 758.3 as the partial pressure of the gas.
  7. Feb 27, 2006 #6
    Thanks.. that might help

    would that mean that the vapor pressure of the water would be 1.7 torr? That seems incredibly low. I'm assuming that total pressure = Pressure(gas) + pressure(water)

    760 = 758.3 + Pwater

    .. this doesnt seem right. That is really low.

    hm... I don't know why this is so tough.


  8. Feb 27, 2006 #7
    It is not that unusual, water is a liquid and so the particles are not moving around as much as they are in its gaseous state of steam.
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