van der waals constants for real gases


I don't understand how to calculate exact constant - b, given only the chemical formula and nothing else.

Q: T || F --- the following molecules are arranged from largest to smallest b van der Waals constant per mol

C2F4, C2H6O, Cl2, Ne

I know that the Van der Waals eq. is a better description of the behavior of real gases and that it is $$ [P + a (n/V)^2] (V/n - b) = RT $$

and that b represents the correction term for the size of the atoms/molecules [=] "units of " volume/mol

I just don't understand how to calculate exact b, given only the chemical formula and nothing else.

After looking at the website could reason a bit through electronegativity that C2F4, must be the largest --- am I supposed to plug in fixed values for P, V and T.

like give P, V, and T any values? for all calculations?

  • 1
    $\begingroup$ The van der Waals constants $a$ and $b$ are empirically determined. How would you go about calculating them? $\endgroup$
    – jacob1729
    Jan 31, 2019 at 14:40
  • 2
    $\begingroup$ I don't think there is a simple relationship between chemical formula and van der Waal's $b$. For example I would expect organic componds which are isomers of each other to have different values for $b$ dispite having the same chemical formula. However the values for $b$ for all the componds in your question are given in the site you linked to. $\endgroup$ Jan 31, 2019 at 14:43
  • $\begingroup$ This is a problem for class and it says to show work. - so I think there's supposed to be a relationship? $\endgroup$ Jan 31, 2019 at 14:44

2 Answers 2


Emailed my TAs and they said this: ( for anyone who comes across this later)

Ranking them by size is the right idea. To mathematically estimate the size of each molecule you may assume each element has a spherical volume, and that the volume of the molecule is the sum of it elements. Google is your friend for known values about the elements [ie. look up the atomic radius and use the normal volume formula].

**Additionally another Physics_Stack Exchange question covers a related concept (linked)

Van der Waals equation**

enter image description here


You cannot calculate the Van der Waals constants. They are determined through experiments. The question doesn't ask you to, it only asks you to compare the Van der Waals constants, which is something that we definitely can do.

Remember that the constant a encodes the attraction between the gas molecules while b encodes both the size of the molecules and the repulsion between the gas molecules. You can use this information to attempt to solve the question, but $C_2H_6O$ is problematic here as it has two isomers, an ether and an alcohol, and the constants will be different for both.

  • $\begingroup$ I can see how the question is a non - valuable one? The TAs who made the hw wanted kids to calculate the constants somehow? I don't remember the original hw. $\endgroup$ Dec 19, 2019 at 20:57
  • $\begingroup$ If it does ask you to calculate the values, I don't know how, you would probably need more information such as the VW radius which would make the calculations trivial. This is the limit of my high school knowledge ¯\_(ツ)_/¯ $\endgroup$ Dec 20, 2019 at 11:59
  • $\begingroup$ This was for a college level thermomdynamics course for engineers. $\endgroup$ Dec 20, 2019 at 19:06

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