Question

What is the mass of a cubic meter of air at 20oC at sea level?

Answer 1

The mass of a cubic meter (density) of air is given as follows (see https://en.wikipedia.org/wiki/Density_of_air#Dry_air):

$$\rho =\dfrac{p}{R^*T}$$

where $p = 1.01\times 10^5Pa$ is the pressure at sea level (average), $T = (273 + 20\degree C) = 293K$ is the temperature, and $R^* = 278J/kg/K$ is the specific gas constant for air. Thus, have:

$$\rho = \dfrac{1.01\times 10^5}{278\cdot 293} \approx 1.24kg/m^3$$

Answer. 1.24 kg.