0
$\begingroup$

van der waals constants for real gases

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?

$\endgroup$

bumped to the homepage by Community yesterday

This question has answers that may be good or bad; the system has marked it active so that they can be reviewed.

  • $\begingroup$ The van der Waals constants $a$ and $b$ are empirically determined. How would you go about calculating them? $\endgroup$ – jacob1729 Jan 31 at 14:40
  • 1
    $\begingroup$ 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. $\endgroup$ – By Symmetry Jan 31 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? $\endgroup$ – ThermoRestart Jan 31 at 14:44
0
$\begingroup$

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

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.