Question

Find the average kinetic energy of oxygen in the air. Assume the air is at a temperature of 21degrees Celsius.

Answer 1

equipartition law says the average Ekin per degree of freedom equals
\[E _{pkin}=\left( 1\over 2 \right) k _{B}.T\]
with
\[k _{B }= 1,3806. 10^{-23} J.K ^{-1}\]
the Boltzmann's constant.
so the total Ekin per gas particle is
\[E _{kin}=\left( 3\over 2 \right) k _{B}.T\]

Answer 2

answer: about
\[6,09 .10^{-21} Joule\]
That is the energy for 1 molecule, approximating for an ideal gas.
If you want it for all the oxygen in the worlds atmosphere, just let me know, I 'll go for it.