Molecular weight is defined as the average molecular weight of the gases in the atmosphere. The molecular weights for pure gases were obtained or calculated from web elements periodic table of the elements. Air on earth at sea level is a mixture of approximately seventy eight percent nitrogen and twenty one percent oxygen with the remaining one percent being a mix of primarily argon plus carbon dioxide, neon, helium and other rare gases, from which the average molecular weight of twenty nine was obtained. Molecular weight is used with temperature and pressure to calculate column height and density of the atmosphere.

column height = 8314 ( J * K / kmol ) * temperature / gravity / molecular weight

density = molecular weight * pressure / 8314 ( J * K / kmol ) / temperature
 

This is used in atmospheric spacecraft, multi stage spacecraft, and spacecraft cost.
  Spacecraft