- #1
dingo_d
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Hello
So I have a lab exam in Joule-Thomson effect and I have given values for a and b for VdW equation of state for nitrogen and carbon dioxide. When I take the critical temp. and pressure and put it in the equations:
[tex]a=\frac{27(T_c R)^2}{64 p_c}[/tex] and
[tex]b=\frac{T_c}{8p_c}[/tex]
b gives correct value, but a is off by a factor of 0.1.
E.g. For [tex]CO_2[/tex], given in the table (the experiments and the Phywe can't be wrong) [tex]a=3.60 Pa\ m^6\ mol^{-2}[/tex], and if I put the values from the http://en.wikipedia.org/wiki/Critical_properties" on wikipedia, I get: [tex]a=3.60\times 10^{-1}\ Pa\ m^6\ mol^{-2}[/tex]. Not only for carbon dioxide, but for nitrogen too...
What is even weirder is that b is correct for the same temps and pressure!
So can someone find where this comes from? I calculated from Zemansky and Dittman the coefficients a and b and they are fine (the same as in wikipedia).
I converted all the necessary values (atm in Pa, used temp in K)...
So I have a lab exam in Joule-Thomson effect and I have given values for a and b for VdW equation of state for nitrogen and carbon dioxide. When I take the critical temp. and pressure and put it in the equations:
[tex]a=\frac{27(T_c R)^2}{64 p_c}[/tex] and
[tex]b=\frac{T_c}{8p_c}[/tex]
b gives correct value, but a is off by a factor of 0.1.
E.g. For [tex]CO_2[/tex], given in the table (the experiments and the Phywe can't be wrong) [tex]a=3.60 Pa\ m^6\ mol^{-2}[/tex], and if I put the values from the http://en.wikipedia.org/wiki/Critical_properties" on wikipedia, I get: [tex]a=3.60\times 10^{-1}\ Pa\ m^6\ mol^{-2}[/tex]. Not only for carbon dioxide, but for nitrogen too...
What is even weirder is that b is correct for the same temps and pressure!
So can someone find where this comes from? I calculated from Zemansky and Dittman the coefficients a and b and they are fine (the same as in wikipedia).
I converted all the necessary values (atm in Pa, used temp in K)...
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