# How do you determine if a type of gas is less dense than air?

I kn that ammonia gas is less dense than air, but is there any rule to kn whether a kind of gas is less dense than air, says, by their molecular mass or bonds between the molecules?? Thx a lot :D

If you can consider the gas as a perfect one, provided you are far from the critical pressure and temperature of the gas (for example, 31C and 73 bar for CO2), you can use following formula derived from Marriotte's law PV = nRT

Ro (specific mass) = (P x M) / (R x T)

where (sorry, I am French and only use international units system) :

P = pressure in Pa M = molar mass in kg/mole of gas (given by tables) R = 8.314 J/K (constant value) T = temperature in K

So Ro will be expressed in kg/m3

But the gas density (say d, the parameter of your request) is actually : Ro / (Ro of air at same pressure and temperature) and you have to calculate Ro(air) the same way, knowing that M(air) is 0.029 kg/mole

If your gas is a mixture, you can consider that : M = SUM (Xi x Mi) where Xi are the mass fraction and Mi the molar mass of each component i of the mixture.