Molecular weight of dry air

For combustion calculations, it is acceptable to take the average molecular weight of dry air to be 28.97, and to assume a simplified composition of 79.0 niole% nitrogen and 21.0 mole% oxygen (or equivalently, 76.8 wt% nitrogen and 23.2 wt% oxygen). 

The relative vapor density (RVD) values in Table 18.9 have been calculated as the density of dry air saturated with the compound of interest at 20°C. This represents the weighted mean molecular weight of the compound-saturated air relative to the mean molecular weight of dry air, which is 29 g/mol. The RVD value may be calculated from Equation 18.23 ... 

An open vessel containing 0.205 Ibm of liquid water is placed in an empty room 5 ft wide, 4 ft deep, and 7 ft high, which initially contains dry air at 90 F. All the water evaporates without changing the room temperature. Use the psychrometric chart to estimate the final relative humidity, wet-bulb temperature, humid volume, dew-point temperature, and specific enthalpy of the room air. Take the molecular weight of dry air to be 29.0. and for simplicity, assume the mass of dry air in the room stays constant at its initial value. 

What is the apparent molecular weight of dry air Use the assumptions and values given in Exercise 4.1. 

The molecular weight of dry air (MWair) is generally assumed to be 28.96, as derived from Table 10.1. 

The specific gravity, SG, is the ratio of the density of a given gas to the density of dry air at the same temperature and pressure. It can be calculated from the ratio of molecular weights if the given gas is a perfect gas. 

The analysis of dry atmospheric air varies with location, altitude, time of year, and other factors. Table 4.4 gives the molecular weights of the constituents and the volumetric and gravimetric analyses. 

A variation of the lamellar model was recently proposed by Haubold et al., in which synchrotron SAXS studies were performed on acid form Nafion 117 samples. The molecular weight of these samples was reported to be 250 000 g mol and the experiments were conducted on dry samples in air and samples equilibrated with water, methanol, and a range of water/methanol mixtures using an in situ flow cell. The most fundamental result of this study is that the data show the usual ionomer peak at = 1.4 nm which gives a Bragg spacing of d= Zjtlq ... 

EXAMPLE 3-4 Dry air is a gas mixture consisting essentially of nitrogen, oxygen, and small amounts of other gases. Compute the apparent molecular weight of air given its approximate composition. 

Fig. 9.10. Apparatus for the determination of molecular weights of air-sensitive compounds by the isothermal distillation technique. Temperature fluctuations of the two solvents are minimized in this illustration by a Dewar filled with water. This apparatus is used in the following manner. In a dry box the sample is placed in a tared tube through sidearm A, the sidearm is then sealed off, the tube and remnant of the sidearm are weighed, and the tube is attached to the apparatus by glassblowing at B. A weighed portion of a standard is introduced into the other bulb, and the filling tube is sealed off. After evacuation, opening of the break-seal, and reevacuation, a measured portion of solvent is distilled into both arms of the apparatus. The process of equilibration is followed by periodic removal and measurement of the solvent from one arm. The solvent may be measured volumetricly in the liquid or gas stales, or by weight.

As can be seen, the flux for heat conduction across the air boundary layer is proportional to / au(7 surf 7"ta) for all three shapes considered (pea = 7s111 for Eq. 7.14).2 Because the conduction of heat in a gas phase is based on the random thermal motion of the molecules, the composition of air, such as its content of water vapor, can influence Kau. Air can hold more water vapor as the temperature increases in that regard, decreases as the water vapor content increases because H2O has a lower molecular weight (18) than is the average for air, which is mainly N2 and O2 (molecular weights of 28 and 32, respectively). For instance, at 20°C Kait at a pressure of 1 atm and 100% relative humidity is 1% less for than it is for dry air, and at 40°C, Km is then 2% lower (Appendix I). 

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

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