Molecular weight of water vapor

Absolute humidity H equals the pounds of water vapor carried by 1 lb of diy air. If ideal-gas behavior is assumed, H = M p/[M P — p)], where M,, = molecular weight of water = molecular weight of air p = partial pressure of water vapor, atm and P = total pressure, atm. 

By replacing the mole fraction of water with the ratio of water vapor pressure (Pw) divided by the total gas pressure (PT), one can solve for the diffusive flux of water vapor. Also, by multiplying Nw by the molecular weight of water, the mass flux of water vapor is arrived at ... 

The molecnlar weight of water vapor (MW= 18) is less than that of air (MW= 29). As snch, the diffnsion of water vapor into the surrounding atmosphere, which consists of a mixtnre of water and air, leads to a buoyant force with upward macroscopic movement. The natural evaporation phenomenon is not only the effect of heat transfer but also a buoyancy-induced motion. The system is at steady state when the vapor pressure of the water at the surface is less than that in the air above, and the resulting condensation is governed by the slow process of molecular diffusion and lamilar flow. 

P = water vapor pressure, air pressure, or total mixture pressure (water plus air) M = molecular weight of the vapor phase... 

Solution. It is convenient to solve this problem in steps. Let us first find out how many moles of water were produced. The number of moles of water produced is found by dividing 0.1598 by 18.02, the molecular weight of water it is 0.00887. Each mole of water vapor contains two gram-atoms of hydrogen hence the number of gram-atoms of hydrogen in the original sample is twice this number, or 0.01774. 

It is therefore possible to calculate the amount of each component in a particular volume of moist air, simply by knowing the water vapor pressure. For instance, in saturated air at 50 C, given that Pwmer is 0.1217 atm, it follows that Pair is 1 - Pwater or 0.8783 atm. The molecular weight of water is O.OlSkg/mol, and that of air is 0.029kg/mol. The masses (m) of water and of air in a I m (10 L) sample of saturated air at 50 C are, therefore ... 

If the absolute humidity, h, is known, the molecular weights of water (18) and bone dry air (29) must be used to convert fi om a mass ratio to a molar ratio. The molar ratio of water vapor to BDA is... 

Humidity is a measure of the amount of water vapor in a gas that can be a mixture, such as air, or a pure gas, such as nitrogen or argon. Based on measurement techniques, the most commonly used units for humidity measurement are relative humidity (RH), dew/frost point (D/F PT), and parts per million (ppm) (Chen and Lu 2005). Relative humidity (RH) is the ratio of the partial pressure of water vapor present in a gas to the saturation vapor pressure of the gas at a given temperature. RH is a function of temperature, and, thus, it is a relative measurement. The RH measurement is expressed as a percentage. Dew point is the temperature (above 0 °C) at which the water vapor in a gas condenses to hquid water. Frost point is the temperature (below 0 °C) at which the vapor condenses to ice. D/F PT is a function of the pressure of the gas but is independent of temperature and is therefore an absolute humidity measurement. Parts per milhon (ppm) represents water vapor content by volume fraction (ppmv) or, if multiplied by the ratio of the molecular weight of water to that of air, as ppmw. PPM is also an absolute measurement. Although this measurement unit is more difficult to conceive, it has extensive apphca-tions in industry, especially for trace moisture measurement. Figure 18.1 shows the correlation between relative humidity (RH), parts per milhon by volume (ppmv), and dew/frost point (D/F PT). 

At pressures below 5.5 atm, the vapor phase normally may be considered ideal. In such a situation, Equations 7-2 and 7-8 apply. Certain systems, such as acetic acid and water, form dimers in the vapor phase. In these cases, the degree of dimerization determines not only the VLE, but the average molecular weight of the vapor. This, in turn, controls the gas density used in the hydraulic capacity calculations. 

Sohd sorbent materials have the abiUty to adsorb water vapor until an equiUbrium condition is attained. The total weight of water that can be adsorbed in a particular material is a function of the temperature of the material and of the relative humidity of the air (see Adsorption). To regenerate the sorbent, its temperature must be raised or the relative humidity lowered. The sohd sorbents most commonly used are siUca (qv), alumina (see Aluminum compounds), and molecular sieves (qv). 

Chlorine heptoxide is more stable than either chlorine monoxide or chlorine dioxide however, the CX C) detonates when heated or subjected to shock. It melts at —91.5°C, bods at 80°C, has a molecular weight of 182.914, a heat of vapori2ation of 34.7 kj/mol (8.29 kcal/mol), and, at 0°C, a vapor pressure of 3.2 kPa (23.7 mm Hg) and a density of 1.86 g/mL (14,15). The infrared spectmm is consistent with the stmcture O CIOCIO (16). Cl O decomposes to chlorine and oxygen at low (0.2—10.7 kPa (1.5—80 mm Hg)) pressures and in a temperature range of 100—120°C (17). It is soluble in ben2ene, slowly attacking the solvent with water to form perchloric acid it also reacts with iodine to form iodine pentoxide and explodes on contact with a flame or by percussion. Reaction with olefins yields the impact-sensitive alkyl perchlorates (18). 

M, = average molecular weight of vapor, dimensionless Po = partial pressure of vapor at the condensate film, °F Po = partial pressure of vapor in gas body, atm L = temperature of condensate film, °F tg = temperature of dry gas (inerts), °F L = temperature of water, °F = latent heat of vaporization, Btu/lb... 

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