Gases have weight — a litre of the air around you carries about $1.2\ \mathrm{g}$. The recipe: $\rho = \dfrac{PM}{RT}$ — read aloud rho equals P M over R T, where $\rho$ is the Greek letter rho, said “row”, and $M$ is the molar mass in g/mol. It is the ideal gas law with the mole count traded for grams: heavier particles at the same crowding weigh more per litre. With P in kPa and $R = 8.314$, the answer lands directly in grams per litre.

And this formula is WHY helium floats: air averages about $29\ \mathrm{g/mol}$, helium weighs in at 4. Same temperature, same pressure, same particle crowding — one-seventh the freight. Anything with $M$ below air's 29 rises; anything above it sinks and pools. Keep 1.2 g/L in your pocket as the sanity rail for every answer below.