Vapour density
Encyclopedia
Vapour density is the density
of a vapour in relation to that of hydrogen
. It may be defined as mass of a certain volume of a substance divided by mass of same volume of hydrogen.
vapour density = mass of n molecules of gas / mass of n molecules of hydrogen
Therefore:
vapour density = molar mass of gas / molar mass of H2
vapour density = molar mass of gas / 2.0016
vapour density = ~½ × molar mass
(and thus: molar mass = ~2 × vapour density)
With this definition, the vapour density would indicate whether a gas is denser (greater than one) or less dense (less than one) than air. The density has implications for container storage and personnel safety—if a container can release a dense gas, its vapour could sink and, if flammable, collect until it is at a concentration sufficient for ignition. Even if not flammable, it could collect in the lower floor or level of a confined space and displace air, possibly presenting a smothering hazard to individuals entering the lower part of that space.
It is recommended that individuals confirm the definition of vapour density being used in their specific context.
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
of a vapour in relation to that of hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
. It may be defined as mass of a certain volume of a substance divided by mass of same volume of hydrogen.
vapour density = mass of n molecules of gas / mass of n molecules of hydrogen
Therefore:
vapour density = molar mass of gas / molar mass of H2
vapour density = molar mass of gas / 2.0016
vapour density = ~½ × molar mass
(and thus: molar mass = ~2 × vapour density)
Alternative definition
In many web sources, particularly in relation to safety considerations at commercial and industrial facilities in the U.S., vapour density is defined with respect to air, not hydrogen. Air is given a vapour density of one. For this use, air has a molecular weight of 29 atomic mass units, and all other gas and vapour molecular weights are divided by this number to derive their vapour density. For example, acetone has a vapour density of 2 in relation to air. That means acetone vapour is twice as heavy as air. This can be seen by dividing the molecular weight of Acetone, 58.1 by that of air, 29, which equals 2.With this definition, the vapour density would indicate whether a gas is denser (greater than one) or less dense (less than one) than air. The density has implications for container storage and personnel safety—if a container can release a dense gas, its vapour could sink and, if flammable, collect until it is at a concentration sufficient for ignition. Even if not flammable, it could collect in the lower floor or level of a confined space and displace air, possibly presenting a smothering hazard to individuals entering the lower part of that space.
It is recommended that individuals confirm the definition of vapour density being used in their specific context.