Using the relationship of density (ρ) of the substance at temperature (T)
We have ρ = Mp/RT
or p = ρ RT/ M
For the given data if M is the molar mass of the gaseous oxide, we have
2 = ρ RT/ M ……………(1)
Also for nitrogen 5 = ρ RT/28 ………………….(2)
From (1) & (2), we have
5/2 = M/28
Or
M = 5 X 28/ 2 = 70g/mol
Hence, the molecular mass of the oxide is 70 g/mol.