preface to fifteenth edition

PHYSICAL PROPERTIES 5.171
This equation requires only knowledge of the critical temperature and pressure for its use and gives
accurate results in the vicinity of room temperature for unassociated substances at moderate pressures.
5.9.1.2 Properties of Gas Molecules
Vapor Density. Substitution of the Antoine vapor-pressure equation for its equivalent log P in
the ideal gas equation gives
log
vap
B
log M log R log (t 273.15) A
t C
where vap is the vapor density in g·mL 1 at tC, M is the molecular weight, R is the gas constant,
and A, B,and C are the constants of the Antoine equation for vapor pressure. Since this equation is
based on the ideal gas law,it is accurate only at temperatures at which the vapor of any specific
compound follows this law. This condition prevails at reduced temperatures (T R ) of about 0.5 K.
Velocities of Molecules. The mean square velocity of gas molecules is given by
3kT 3RT
2
u
m M
where k is Boltzmann’s constant and m is the mass of the molecule.
The mean velocity is given by
8u 2 1/2
u
3
Viscosity.
On the assumption that molecules interact like hard spheres,the viscosity of a gas is
5 mkT
1/2
2
16
where is the molecular diameter.
Mean Free Path. The mean free path of a gas molecule l and the mean time between collisions
are given by
l
m
2
p2
1 4
u 5P
Graham’s Law of Diffusion. The rates at which gases diffuse under the same conditions of
temperature and pressure are inversely proportional to the square roots of their densities:
1/2
r1 2
r
2 1
Since MP/RT for an ideal gas,it follows that
r1 M2
r M
2 1
1/2