what is the density of atmosphere assuming that O2 : 21% and N2 : 79%

Question

I just can't seem to work this out, and I was thinking whether I use the ideal gas law, and find the empirical formula, but again, I have no idea.

Answer 1

Density is given as mass/volume for a gas, it will depend on the pressure and temperature.

If we assume STP where T = 273K and P = 1 atm, we can calculate the volume given the moles of gas.

If you had 100 g of gas...

21% O2 = 21 g O2 x 1 mol/32 g = 0.66 moles O2

70% N2 = 79 g N2 x 1 mol/28 g = 2.82 moles N2

Total moles of gas = 3.48 moles

PV = nRT

V = nRT/P = (3.48)(0.0821)(273)/1 = 78 liters = 78,000 mls

density = mass/volume = 100 g/78,000 ml = 0.00128 g/ml = 1.28 g/L etc....