The vapor density of a gas is the mass of a unit volume of the gas compared to the mass of a unit volume of hydrogen gas at the same temperature and pressure.  It can also be expressed as the ratio of the density of a gas to the density of hydrogen at the same temperature and pressure.

The mass of a unit volume of the gas compared to equal volume of water vapour

The mass of a unit volume of the gas compared to equal volume of hydrogen

The mass of a unit volume of the gas compared to equal volume of oxygen

The mass of a unit volume of the gas minus the vapour pressure of water

The vapour density of a gas may be defined as

The vapour density of a gas may be defined as

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