Given Information:

Temperature of gas is 20 ° C

Number of molecules is 1012 per cubic millimeter.

The mass of single molecule of air calculated as

m =Molecular weight Avogadro's number =28.97 mol-16.023E23 molecules/g.mol=4.81E-23 g

Then the density of air including 10¹² molecules per mm³ is, in SI units,

ρ=(1012molecules mm 3)(4.81E-23( g molecule))=4.81E-11gmm3=4.81E-5 kgm3

Conclusively, from the perfect gas law, at 20 °C  293 K, we get the pressure:

p=ρRT=(4.81E-5 kg m 3)(287 ( m2 s2 K))(293​ K)=4.0​ Pa

Conclusion:

Therefore, the absolute pressure (in Pa) for air is 4.0 Pa.