Gaseous and Liquid Phases of Homopolar Substances

Gas Formula a h Vr/3 Molecular volume of liquid Electric moments 

The unit of pressure in the constants above is the dyne per square centimeter, the unit of volume is cubic centimeters per mole. The electric moments are expressed in absolute electrostatic units. Data for Van der Waals constants and volumes are taken from Landolt s Tables for the electric moments from Debye, Polare Molekeln, Leipzig, 1929. 

The actual magnitude of the constants b is of considerable interest. In the first place, we may ask how much of the volume of a gas, under ordinary conditions of pressure and temperature, is occupied by the molecules. One mole of a gas at atmospheric pressure and 0°C. occupies 22.4 1., or 22,400 cc. The molecules, under the same circumstances, occupy the volume tabulated in Table XXIV-1, in cubic centimeters. For the common gases, these volumes are of the order of 30 or 40 cc. This is the order of magnitude of two-tenths of 1 per cent of the actual volume, so that it is correct to say that most of the volume occupied by a gas is really empty. On the other hand, even under these circumstances, the 

From what we have said, the values of the Van der Waals constants b for the gases of our table look very reasonable. Next we can consider their a s. In Eq. (3.6) of Chap. XXII, we have seen that the Van der Waals interaction energy between molecules of polarizability a, mean square dipole moment /, at a distance r, is 

we have seen that the Van der Waals a for molecules of radius r0/2, with an intermolecular attractive potential of f , is 

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