Question

calculate the number of molecules in 1 cm cube of oxygen at normal temperature and atmospheric pressure

Answer 1

1 cm3 = 1 ml
moles of oxygen = 1ml/22400ml
molecules of oxygen = (1/22400)×6.022×10^23.

Answer 2

PV = nRT
n = PV/(RT)
n = 1 * 10^-3 / ((22.4/273) * 273)
n = 1/22400 mol

Number of molecules = 6 * 10^23 / (22400) = 2.67 * 10^19 molecules