#format jsmath
= Mean Free Path =
From CRC100:

|| $l$    || mean free path          || m      ||
|| $d$    || hard sphere diameter    || m      ||
|| $P$    || pressure                || Pa|    ||
|| $\eta$ || viscosity               || μPa s  ||
|| $m$    || molecular mass          || kg     ||
|| $M$    || molar mass              || g/mole ||
|| $\tau$ || mean time between collisions || s ||
|| $\overline v$ || mean velocity   || m/s    ||

$ l = { \Large { { k T } \over { \pi \sqrt{ 2 } P d^2 } } } ~ = ~ { \Large { 9.27e-27 \over { d^2 } } } $

$ d^2 = 2.669E-20 \sqrt( M T ) / \eta ~~~ $ m² ... compute diameter from viscosity

$ \overline{ v } = { \Large \left( { 8 k T } \over { \pi m } \right) } = 145.5\sqrt(T/M) ~~~ $ m/s

At 25C and 100KPa  (or STP?):

|| gas || d || l || $\overline v$ || $\tau$ || $\eta$ ||
||     || Å    || nm   ||  m/s || ps || μPa s ||
|| NH₃ || 4.32 || 49.7 ||  609 ||  82 || 10.2 ||
|| Ar  || 3.58 || 72.2 ||  397 || 182 || 22.7 ||
|| CO₂ || 4.53 || 45.1 ||  379 || 119 || 14.9 ||
|| He  || 2.15 || 200  || 1256 || 159 || 19.9 ||
|| H₂  || 2.71 || 126  || 1769 ||  71 ||  8.9 ||
|| Kr  || 4.08 || 55.8 ||  274 || 203 || 25.4 ||
|| Ne  || 2.54 || 143  ||  559 || 256 || 32.0 ||
|| N₂  || 3.70 || 67.6 ||  475 || 142 || 17.8 ||
|| O₂  || 3.55 || 73.6 ||  444 || 166 || 20.7 ||
|| Xe  || 4.78 || 4.05 ||  219 || 185 || 23.1 ||
|| H₂O ||      ||      ||      ||     ||  9.9 ||
|| CO  ||      ||      ||      ||     || 17.8 ||
|| CH₄ ||      ||      ||      ||     || 11.1 ||

also CRC Handbook of Thermophysics p407