Physics Gas Law

First calculate the molar mass of air.
In 1 mole of air, there are Na (Avogadro's number = 6.02 x 10^23) air molecules, of which there are 0.21Na molecules of O2 and 0.78Na molecules of N2

Mass of O2 molecules = 0.21Na x (16x2/Na) g = 0.21 x 32 g
[note: both O2 and N2 are diatomic]

Mass of N2 molcules = 0.79Na x (2x14/Na) g = 0.79x 28 g

Total mass of molecules = (0.21x32 + 0.79x28) g = 28.84 g
hence, molar mass of air = 28.56 g/mol
[note: the commonly accepted value for molar mass of air is 28.96 g/mol. The difference is that CO2 and inert gases have not been included in the calculation]
Now, using the ideal gas equation, PV = nRT
the no. of moles of air n = PV/RT = 10^5 x (2.5x4x5)/8.31x(273+25)
i.e. n = 2019

Hence, mass of air = 2019 x 28.84 g = 58228 g = 5.82 kg