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 dimensional analysis- off by 10

  1. Sep 14, 2010 #1
    1. The problem statement, all variables and given/known data
    I am going crazy! The problem tells me to use the ideal gas equation to solve for the specific volume. They give the pressure, 15 MPa, and the temperature, 623 K. However, solving it is not the problem. The problem is that I keep on getting the answer off by a factor of 10^3!

    2. Relevant equations
    P = Pressure
    v = specific volume
    R = gas constant
    T = temperature

    Pv = RT
    v = RT/P

    3. The attempt at a solution

    1. Solve for gas constant

    M (molecular mass) = 0.01802 kg/kmol
    R (universal) = 8.314 (kPa*m^3)/(kmol*K)

    R = ((8.314 kPa*m^3)/ 1 kmol*K) * (1 kmol/.01802 kg)
    R = 461.4 (kPa*m^3)/(K*kg)

    2. Solve for Specific volume

    v = (461.4 (kPa*m^3)/(K*kg))*(623K)/((15*10^3) kPa)

    v = 19.16 (m^3)/kg

    But the answer should be 0.0191 instead of 19.16...
  2. jcsd
  3. Sep 15, 2010 #2


    User Avatar
    Homework Helper

    Which substance has the molar mass of 0.01802 kg/kmol = 0.01802 g/mol? The lightest molar mass (that of H2) is 2 g/mol!

  4. Sep 15, 2010 #3
    I figured out what i did wrong.

    18.02 g/mol = 18.02 kg/kmol!

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook