1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Thermodynamics - Find initial pressure (given Temperatures)

  1. Sep 25, 2010 #1
    1. The problem statement, all variables and given/known data

    For a certain experiment, R-410a vapor is contained in a sealed glass tube at 20°C. It is
    desired to know the pressure at this condition, but there is no means of measuring it, since
    the tube is sealed. However, if the tube is cooled to −20°C small droplets of liquid are
    observed on the glass walls. What is the initial pressure?

    2. Relevant equations

    Thermodynamic tables
    ideal gas equation?

    3. The attempt at a solution

    I set up both states:
    State 2: T = -20 (saturated)

    State 1: T = 20, v1=v2=0.0648 P = ? (superheated)

    I just don't understand how we can figure it out from just the temps. I just cant wrap my mind around the thermodynamic tables.
     
  2. jcsd
  3. Sep 26, 2010 #2

    Borek

    User Avatar

    Staff: Mentor

    What happens at -20?
     
  4. Sep 26, 2010 #3
    At -20, the condensation occurs?
     
  5. Sep 26, 2010 #4

    Borek

    User Avatar

    Staff: Mentor

    Can you use it to find vapor pressure at -20? With the help of tables?
     
    Last edited by a moderator: Aug 13, 2013
  6. Sep 26, 2010 #5
    Is it read directly from the table? would it be 399.6 kPa?
     
  7. Sep 26, 2010 #6

    Borek

    User Avatar

    Staff: Mentor

    No idea, I don't have these tables. But yes, you should read it directly from the table.
     
    Last edited by a moderator: Aug 13, 2013
  8. Sep 26, 2010 #7
    So, how would I use this information to find the initial pressure?

    Could I just use the ideal gas? P1/T1 = P2/T2 ?
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook