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!

Work done on steam during phase change?

  1. Aug 30, 2012 #1
    1. The problem statement, all variables and given/known data
    (a) How much work is done on the steam when 3.89 mol of water at 100°C boils and becomes 3.89 mol of steam at 100°C at 1.00 atm pressure? (Assume the latent heat of vaporization of water is 2.26 106 J/kg.)

    (b) Assume the steam to behave as an ideal gas. Determine the change in internal energy of the system of the water and steam as the water vaporizes.


    2. Relevant equations

    Q + W = ΔE
    Q = m * Lv
    W = -P * ΔV = -P * (V2 - V1)
    P * V = n * R * T

    3. The attempt at a solution

    Convert moles of water to volume (multiply by molar mass, then use density of water)
    Volume of water: 0.07002 m^3.

    Then find the volume of steam using ideal gas law: V = ((3.89 mol) * (8.31 J / mol K) * (373.15K)) / (101325 Pa)
    Volume of steam: 0.1190 m^3

    So work should be -P * (V2 - V1) -> -(101325 Pa) * ((.1190 m^3) - (.07002 m^3))

    W = -4963 J = -4.963 kJ.

    But my web assign homework tells me that I am over 10% away from the correct answer.

    I can't do part (b) until I can complete part (a) because W + Q = ΔE, so without knowing the work, I can't find the change in internal energy (at least that's what my prof said when I asked him).

    Thanks in advance for any help :)
     
  2. jcsd
  3. Aug 30, 2012 #2

    TSny

    User Avatar
    Homework Helper
    Gold Member

    Check where the decimal point should go here.
     
  4. Aug 30, 2012 #3
    (3.89 mol) * (18 g / mol) = 70.02 g = 70.02 * 10^(-3) kg

    move the decimal over three places: 0.07002 kg, then density of water is 1 kg/ m^3, so:

    .07002 m^3, right?
     
  5. Aug 30, 2012 #4

    TSny

    User Avatar
    Homework Helper
    Gold Member

    It would be very hard to pick up a m3 of water! So, is the density of water 1 kg/m3?
     
  6. Aug 30, 2012 #5
    Oh my gosh, I can't believe I did that XD! Thanks 1000 kg/m^3
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Work done on steam during phase change?
Loading...