vanderwaals correction
Answers
The constants a and b have positive values and are characteristic of the individual gas. The van der Waals equation of state approaches the ideal gas law PV=nRT as the values of these constants approach zero. The constant a provides a correction for the intermolecular forces. Constant b is a correction for finite molecular size and its value is the volume of one mole of the atoms or molecules.
van der Waals Coefficients
Gas
a (Pa m6)
b(m3/mol)
Helium
3.46 x 10-3
23.71 x 10-6
Neon
2.12 x 10-2
17.10 x 10-6
Hydrogen
2.45 x 10-2
26.61 x 10-6
Carbon dioxide
3.96 x 10-1
42.69 x 10-6
Water vapor
5.47 x 10-1
30.52 x 10-6
Data from Fishbane, et al.
Since the constant b is an indication of molecular volume, it could be used to estimate the radius of an atom or molecule, modeled as a sphere. Fishbane et al. give the value of b for nitrogen gas as 39.4 x 10-6 m3/mol. This leads to the following estimate of radius