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Air is given a vapour density of one. For this use, air has a molecular weight of 28.97 atomic mass units, and all other gas and vapour molecular weights are divided by this number to derive their vapour density. [2] For example, acetone has a vapour density of 2 [3] in relation to air. That means acetone vapour is twice as heavy as air.
where the pressure, p, is the atmospheric pressure, V is the measured volume of the vessel, T is the absolute temperature of the hot bath, and R is the gas constant. The molecular weight of the chemical is then simply the mass in grams of the vapor within the vessel divided by the calculated number of mole.
This principle is included in the ideal gas equation: =, where n is the amount of substance. The vapour density (ρ) is given by =. Combining these two equations gives an expression for the molar mass in terms of the vapour density for conditions of known pressure and temperature:
How much gas is present could be specified by giving the mass instead of the chemical amount of gas. Therefore, an alternative form of the ideal gas law may be useful. The chemical amount, n (in moles), is equal to total mass of the gas (m) (in kilograms) divided by the molar mass, M (in kilograms per mole): =.
Water vapor and dry air density calculations at 0 °C: The molar mass of water is 18.02 g/mol, as calculated from the sum of the atomic masses of its constituent atoms. The average molar mass of air (approx. 78% nitrogen, N 2; 21% oxygen, O 2; 1% other gases) is 28.57 g/mol at standard temperature and pressure .
As the liquid begins to vaporize, the fluid becomes a heterogeneous mixture of liquid and vapor whose molar volume varies continuously from to according to the equation of state = + where = / (+) and is the mole fraction of the vapor. This equation is called the lever rule and applies to other properties as well.
ρ L is the liquid density in kg/m 3 ρ V is the vapor density in kg/m 3 k = 0.107 m/s (when the drum includes a de-entraining mesh pad) Then the cross-sectional area of the drum can be found from: = ˙ where ˙ is the vapor volumetric flow rate in m 3 /s A is the cross-sectional area of the drum
The Antoine equation [3] [4] is a pragmatic mathematical expression of the relation between the vapor pressure and the temperature of pure liquid or solid substances. It is obtained by curve-fitting and is adapted to the fact that vapor pressure is usually increasing and concave as a function of temperature.