Gases:  Deviations from Ideal Gas Law Behavior: Van der Waals Equation
Van der Waals Constants
for Common Gases
Compound
 
a (L2-atm/mol2)
 
b (L/mol)
He
0.03412
0.02370
Ne
0.2107
0.01709
H2
0.2444
0.02661
Ar
1.345
0.03219
O2
1.360
0.03803
N2
1.390
0.03913
CO
1.485
0.03985
CH4
2.253
0.04278
CO2
3.592
0.04267
NH3
4.170
0.03707
  Conditions are "Ideal" at:            Conditions are "Real" at:

High Temperature                         Low Temperature

Low Pressure                             High Pressure

WHY?
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Relationship between Boiling Point and the "a" Constant

Boiling Point - The temperature at which the vapor pressure of the liquid equals the pressure on the liquid (usually atmospheric pressure).

  • In other words, the temperature at which the molecules have enough energy to escape the forces of attraction that hold a liquid from becoming a gas.
  • [Image]   Since the "a" constant corrects for the existing
    forces of attraction between gas molecules, it is
    easy to understand why there is a correlation 
    between this constant and the boiling point. 
    A substance with a higher boiling point has
    stronger forces of attraction which hold the
    molecules together.  Likewise, this substance
    would have a higher "a" constant value also
    because of these stronger forces of attraction.