Mole fraction partial pressure

In (kmol) (kmol) Out (mole fraction) (partial pressure)... 

Analysis of mass transfer systems is complicated by the range of units in which gas and liquid concentrations may be defined, e.g. mass concentration, molar concentration, mass fractions, mole fractions, partial pressures, etc. This leads to a wide range of units for the mass transfer coefficient. 

The numerical value of kt in (2-3) depends on how activity is defined and on the units in which concentration is expressed (molarity, mole fraction, partial pressure). Measurement of the absolute activity, or chemical potential, of an Individual ion is one of the classical unsolved problems. Since we cannot measure absolute ion activity, we are then necessarily interested in the next best—comparative changes in activities with changing conditions. To obtain comparative values numerically, we measure activity with respect to an arbitrarily chosen standard state under a given set of conditions of temperature and pressure, where the substance is assigned unit activity. The value of ki in (2-3) thus depends on the arbitrary standard state chosen accordingly, the value of the equilibrium constant also depends on the choice of standard states. 

This allows the concentration units to be changed to suit the problem, and other measures of concentration, such as mole fraction, partial pressure, or fugacity, might be introduced, g is a good measure to use for gaseous reactions with volume changes and will be mentioned later in connection with tubular reactors otherwise we will phrase our problems in terms of c, recognizing that this implies no limitation on the method. 

In summary, thermodynamic models of natural water systems require manipulation of chemical potential expressions in which three concentration scales may be involved mole fractions, partial pressures, and molalities. For aqueous solution species, we will use the moial scale for most solutes, with an infinite dilution reference state and a unit molality standard state (of unit activity), l or the case of nonpolar organic solutes, the pure liquid reference and standard states are used. Gaseous species will be described on the partial pressure (atm — bar) scale. Solids will be described using the mole fraction scale. Pure solids (and pure liquids) have jc, = 1, and hence p, = pf. 

Constituent Moles Mole fraction Partial pressure... 

Sample Problem 11.12 lets you practice calculations involving mole fractions, partial pressures, and total pressure. 

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