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### Mean Free Path of Gases Formulae and Calculator

Engineering Materials Specifications and Characteristics Tables and Charts
Thermodynamics and Heat Transfer

Mean Free Path of Gases Formulae and Calculator

The kinetic theory of gases states that molecules can be viewed as spheres which by the energy contained create collisions between them. The approximate mean distance traveled by a molecule between collisions is defined as the mean free path l and is related to the molecule diameter.

The mean free path formulae:

l = ( k · T ) / ( π · P · d2 · 20.5 )

Where:

l = mean free path given in pico meters (pm),
P = pressure (Pa),
T = absolute temperature (K),
d = atomic radius pico meters (pm)
k = Boltzmann constant (1.380649 · 10-23 j / K)
standard conditions (P = 100,000 Pa and T = 298.15 K)

The collusion diameter may be determined from the molecule viscosity and the kinetic theory relation formulae:

η = ( 2.67·10-20 · ( M · T )0.5 ) / d2

Where:

η = Viscosity ( mPa s),
M = molar mass ( g/mol ),

The formulae for the relation of the mean velocity v of molecules of mass m:

v = [ ( 8 · k · T ) / ( π · m ) ]0.5 = 145.5 ( T / m )0.5

Where:

v = mean velocity (m/s)
T = absolute temperature (K),
M = molar mass ( g/mol ),

Mean time τ between collisions formulae:

τ = l / v

 Typical Molecule Diameter and Molar Mass Molecule Diameter m Molar Mass g / mol Air 3.66 ·10-10 28.960 Ar 3.58·10-10 39.948 CO2 4.53·10-10 44.01 H2 2.71·10-10 2.0159 He 2.15·10-10 4.0026 Kr 4.08·10-10 83.800 N2 3.70·10-10 28.014 NH3 4.32·10-10 17.031 O2 3.55·10-10 31.999 Xe 4.78·10-10 131.290 CH4 3.80·10-10 16.043 Ne 2.75·10-10 20.180

References:

1. Reid, R.C., Prausnitz, J.M., and Poling, B.E., The Properties of Gases and Liquids, Fourth Edition , McGraw-Hill, New York, 1987.
2. Lide, D.R., and Kehiaian, H.V., CRC Handbook of Thermophysical and Thermochemical Data, CRC Press, Boca Raton, FL, 1994.
3. Vargaftik, N.B., Tables of Thermophysical Properties of Liquids and Gases, Second Edition, John Wiley, New York, 1975.
4. Kaye, G.W.C., and Laby, T.H., Tables of Physical and Chemical Constants, 15th Edition, Longman, London, 1986.