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Homework Help: Thermodynamics dimensional analysis- off by 10

  1. Sep 14, 2010 #1
    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


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    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!
