Gas Density

Relative density of a gas to hydrogen, air or oxygen, density at STP and molar mass of a gas, with a solution.

What to find:

the densities of a gas from its formula

the molar mass from the relative density

the molar mass from the density at STP

the molar mass from the mass and volume of a gas

The relative density D shows how many times a gas is heavier than another gas: D = M₁ / M₂. The average molar mass of air is 29 g/mol. Write formulas in Latin letters starting with a capital: H2SO4, Ca(OH)2, CuSO4·5H2O (water of crystallization after a dot or *).

Test yourself

The gas density calculator finds the relative density of a gas with respect to hydrogen, air, oxygen and other gases, the density at standard conditions and the molar mass. From the molar mass it suggests which gas it may be.

Formulas

D₂(1) = ^M₁/_M₂, ρ = ^M/_{V_m} = ^M/_22.4 g/L

The relative density shows how many times one gas is heavier than another under the same conditions. The average molar mass of air is 29 g/mol, so M = 29 · D_air.

Gases lighter than air (D_air < 1: hydrogen, helium, ammonia, methane) are collected in an upside-down vessel, and gases heavier than air (oxygen, carbon dioxide, chlorine) in a vessel with the opening up.

Example. CO₂: M = 44, D_H₂ = 44 / 2 = 22, D_air = 44 / 29 ≈ 1.52, ρ = 44 / 22.4 ≈ 1.96 g/L.