# Van der Waals constant $b$ (real gas) chemical form. only

http://www2.ucdsb.on.ca/tiss/stretton/database/van_der_waals_constants.html

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?

• The van der Waals constants $a$ and $b$ are empirically determined. How would you go about calculating them? Jan 31, 2019 at 14:40
• 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. Jan 31, 2019 at 14:43
• This is a problem for class and it says to show work. - so I think there's supposed to be a relationship? Jan 31, 2019 at 14:44

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].

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.