Question

Dry air is about 20.95% oxygen by volume. Assuming STP, how many oxygen molecules are in a 75.0 g sample of air? The density of air is 1.19 g/L.

Answer 1

At STP temperature, T=273.15K, pressure, P=101325Pa.

mass, m=75g and density, d=1.19g/l, R=8.315

number of moles,$n=PV/RT$

$V=m/d$

V=75/1.19= 63cm3

n=(101325*63*10^-6)/(8.315*273.15)

n=2.81*10^-3moles