2.5 m

There's 7.79215x10^26 N2 molecules. A single molecule = 14.007u. So you work out the total mass contained within the subset of consideration: (total number of N2 molecules ) x (2 x weight of single N molecule). Then consider the density of the subset of the mass rho = 0.75518 x 1.2. Then density = mass/ volume => height = mass/(16x1.2x0.75518)