physics problem.. need help?

Question

consider a classroom 10 m long by 6 m wide by 4 m high at "room temp" 20 degrees celcius and 1 atmosphere. What is the mass of room air?if all air was replaced by helium what would be the weight of helium in the room?

Answer 1

m=pV
m=mass
p - density
V - volume

V= 10x6x4=240 m^3
At standard temperature and pressure (0 °C and 101.325 kPa), dry air has a density of ρSTP = 1.292 kg/m3.
m=pV= 1.292 x 240=310 kg

He? p=0.1786 g/L or 0.1786 kg/m^3

m=0.1786 x 240= 42.9 kg