Question

density of dry air containing only N2 and O2 is 1.15 g/l at 740mm of hg and 300 k what is the % composition of N2 by mass in the air

Answer 1

PV = n RT= (m/M) RT
Where P is pressure,
V is volume,
n is number of moles
R is a constanet and
T is the kelvin temperature,
mm is the mass of gas and M is the molecular weight of the gas.
Now we know that
M= dRT /P
=1.15 x .0821 x 300/ 740 /760 = 29.07gmol-1
Now take W1 is the mass fraction of nitrogen and 1-W1 is the mass fraction of oxygen
Now we get W1*28 + (1 - W1)32 = 29.07
Now solving the above equation we get
-4W1 = -2.94
So W1 = 0.736
So % of nitrogen is 73.6 by weight of the air.