# 09 - T hermodynamic Properties of I deal Gases and Ideal...

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Thermodynamic Properties of Ideal Gases and Ideal Gas Mixtures Constant Corn position It has been shown that it is necessary to use quite sophisticated equations of state to define the properties of vapours which are close the saturated vapour line. However, for gases in the superheat region, the ideal gas equation gives sufficient accuracy for most purposes. The equation of state for an ideal gas, in terms of mass, is pV=mRT (9.1) where p = pressure (N/m2) v = volume (m3> m = mass (kg) R = specific gas constant (kJ/kg K) T = absolute (or thermodynamic) temperature (K) This can be written in more general terms using the amount of substance, when pV= n9lT (9.2) where n = amount of substance, or chemical amount mol) 9l = universal gas constant (H/kmol K) It is found that eqn (9.2) is more useful than eqn (9.1) for combustion calculations because the combustion process takes place on a molar basis. To be able to work in a molar basis it is necessary to know the molecular weights (or relative molecular masses) of the elements and compounds involved in a reaction. 9.1 Molecular weights ne molecular weight (or relative molecular mass) of a substance is the mass of its molecules relative to that of other molecules. The datum for molecular weights is carbn- 12, and this is given a molecular weight of 12. All other elements and compounds have molecular weights relative to this, and their molecular weights are not integers. To be able to perform combustion calculations it is necessary to know the atomic or molecular

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State equation for ideal gases 159 weights of commonly encountered elements; these can be combined to give other compounds. Table 9.1 gives the data for individual elements or compounds in integral numbers (except air which is a mixture of gases); in reality only carbon-12 (used as the basis for atomic/molecular weights) has an integral value, but most values are very close to integral ones and will be quoted as such. Table 9.1 Molecular weights of elements and compounds commonly encountered in combustion - Atmospheric Air 0, N, N, H2 CO CO, H,O C m, 28.97 32 28 28.17 2 28 44 18 12 9.1 .I AIR As stated previously, most combustion takes place between a hydrocarbon fuel and air. Air is a mixture of gases, the most abundant being oxygen and nitrogen with small proportions of other gases. In fact, in practice, air is a mixture of all elements and compounds because everything will evaporate in air until the partial pressure of its atoms, or molecules, achieves its saturated vapour pressure. In reality, this evaporation can usually be neglected, except in the case of water. Table 9.1 shows that the molecular weight of atmospheric nitrogen is higher than that of pure nitrogen. This is because ‘atmospheric nitrogen’ is taken to be a mixture of nitrogen and about 1.8% by mass of argon, carbon dioxide and other gases; the molecular weight of atmospheric nitrogen includes the effect of the other substances.
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## This note was uploaded on 03/09/2010 for the course MECHANICAL ME9802701 taught by Professor Prof.william during the Spring '10 term at Institut Teknologi Bandung.

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09 - T hermodynamic Properties of I deal Gases and Ideal...

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