If you assume the molecules have zero volume, then it acts as a "perfect gas" and will follow Charles' Law (

http://en.wikipedia.org/wiki/Charles%27_law).

This means that its volume will shrink from "Gas at 298 K d = 0.657 kg/m^3" down to zero volume at absolute zero (0K=-273.16C).

Of course, it stops acting as an ideal gas when you approach the boiling point (111K).

Van der Waals produced a more accurate relationship which takes into account the finite volume of the molecule, and the attraction between the molecules (

http://en.wikipedia.org/wiki/Van_der_Waals_equation). This gives results which are more accurate than a "straight line" extrapolation to absolute zero.

Van der Waals' formula requires two parameters:

- a is a measure for the attraction between the particles, and
- b is the average volume excluded from v by a particle

However, I am having trouble finding the a & b values for methane - could an experienced chemist show where to find these quantities, please?