Density and Specific Gravity of Gases

Based on the equation of state for an ideal gas, it is possible to establish the theoretical equation of the mass density of a gas denoted by the Greek letter p and expressed in kg.m having a molar mass M as a function of the absolute pressure P and temperature T. Actually, the density being the ratio of the mass of the gas divided by the volume it occupies  

The above equation indicates that the mass density of an ideal gas under isothermal and isobaric conditions is only related to its molar atomic or molecular mass. 

Usually, engineers utilize the specific gravity or relative density of gases, denoted d or S.G. This dimensionless physical quantity refers to the ratio of the mass density of the gas over that of a reference gas, usually dry air, measured under normal conditions of temperature and pressure (NTP). For ideal gases, the specific gravity relative to dry air at the same temperature and pressure can be written as the ratio of their molar masses  

Therefore, in practice, knowledge of the molar atomic or molecular mass of a gas allows us to estimate its specific gravity with respect to dry air quickly conversely, the measurement of the specific gravity of a gas with respect to air under known T and P conditions allows the determination of its molar mass. 

In the case of real gases at low pressure that satisfy the simplified virial equation of state, i.e., PV=n[RT + BpP ], the mass density is given by the following equation  

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