Because the density of a substance is defined as mass per unit volume, the density of gas, ρg, at a given temperature and pressure can be derived as follows:
where
Mw = molecular weight
Because it is more convenient to measure the specific gravity of gases than the gas density, specific gravity is more commonly used. Specific gravity is defined as the ratio of the density of a gas at a given temperature and pressure to the density of air at the same temperature and pressure, usually near 60°F and atmospheric pressure. Whereas the density of gases varies with temperature and pressure, the specific gravity is independent of temperature and pressure when the gas obeys the ideal gas law. By the previous equation, the density of air is
Then the specific gravity, γg, of a gas is
Equation (2.6) might also have been obtained from the previous statement that 379.4 ft3 of any ideal gas at 14.7 psia and 60°F is 1 mol and therefore a weight equal to the molecular weight. Thus, by definition of specific gravity,
If the specific gravity of a gas is 0.75, its molecular weight is 21.7 lbs per mol.
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