Page 191 - Advanced thermodynamics for engineers

CHAPTER

THERMODYNAMIC PROPERTIES 9

OF IDEAL GASES AND IDEAL GAS

MIXTURES OF CONSTANT

COMPOSITION

It was shown in Chapter 8 that it is necessary to use quite sophisticated equations of state to define the
properties of vapours which are close to 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 ¼ mRT (9.1)
where
2
p ¼ pressure (N/m )
3
V ¼ volume (m )
m ¼ mass (kg)
R ¼ specific gas constant (kJ/kg K)
T ¼ absolute (or thermodynamic) temperature (K)
This can be written in more general terms using the amount of substance, when
pV ¼ n<T (9.2)
where

n ¼ amount of substance, or chemical amount (kmol)
<¼ universal gas constant (kJ/kmol K)
Equation (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 on 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
The 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 carbon-12, and this is given a molecular
weight of 12. All other elements and compounds have molecular weights relative to this, and their

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