Mole fraction, usage

Predictive method results are still compared to the Deaton and Frost data. It should be remembered, however, that while this study was both painstaking and at the state-of-the-art, the data were of somewhat limited accuracy, particularly the measurements of gas composition. As will be seen in Chapters 4 and 5, small inaccuracies in gas composition can dramatically affect hydrate formation temperatures and pressures. For example, Deaton and Frost were unable to distinguish between normal butane and iso-butane using a Podbielniak distillation column, and so used the sum of the two component mole fractions. Accurate composition measurement techniques such as chromatography did not come into common usage until early in the 1960s. 

Solution composition can also be described by the number of moles of solute and solvent. The mole fraction of a solution component, then, is the ratio of the number of moles of that component divided by the total number of moles. We could multiply a mole fraction by 100% and speak of mole %, but we usually avoid this usage to avoid confusion with mass %. Thus if a solution is described in terms of % composition, you can assume mass %. 

IUPAC recommend that the mole should be replaced by amount of substance. Readers should however be aware of the wide usage of the concept of the mole elsewhere. The term mole fraction, x, is a IUPAC acceptable name. The Term amountfraction is not recommended. 

Randall, while Prigogine and Defay, in the French edition, adopt convention (c). These two usages have in effect been criticized by Guggenheim on the grounds that unless the standard pressure and standard temperature are universally recognized, then whenever, for example, the term standard is employed as in (c) some ambiguity exists unless the values of the standard pressure and temperature are stated. For this reason in the present translation the term standard has been restricted to (a) anymore detailed specification is given in full, for example the standard chemical potential at one atmosphere and at 600 °K jit (600 °K 1 atm.). The only implication in this is that the composition of the system is to be expressed in mole fractions. As will be seen in chap. XX, when other concentration units are used, a different superscript is employed e.g. p) for molalities and p) for molar... 

Mole fraction is the conventional measure of composition. We use the generic symbol x, to denote this quantity when no particular phase (solid, liquid, or gas) is implied. When referring to a specific phase, we use common noiatien. for example, x, for liquid-phase mole fraction and y, for the vapor-phase mole fraction. The dual usage of x, should cause no confosion because it will he clear from the context whether an arbitrary phase or a liquid phase is under consideration. 

All of the processes for separating isotopes of hydrogen or other light elements dealt with in this chapter involve distribution between a liquid and a vapor phase. To remain consistent with standard chemical engineering usage, component fractions in the vapor phase are denoted by y and the liquid phase by x. For a two-component mixture, the symbol y or x wiD denote the fraction of desired component (e.g., atom fraction deuterium in a mixture of HjO, HDO, and DjO) in the vapor or liquid phase. For a mixture containing three or more components, a subscript will be used to designate the component. For example, hd denotes mole fraction HD in a vapor mixture of H2, HD, and Dj. However, in mixtures of Hj, HD, and D2 whose deuterium content is so low that the fraction of D2 can be ne ected, the mole fraction of HD will be denoted by > or x without subscript. 

Because the activities of solutes in dilute solutions can be more closely approximated with Henry s Law than with Raoult s Law, they are traditionally treated separately and use standard states different than those we have so far encountered. There are two variations of usage here, both of the variable pressure type, one required when using mole fractions, and another when using molalities. 

The analogy is with deviations from Raoult s law for vapor-liquid equilibria, where ideally the vapor-phase partial pressure for each component of a mixture is equal to the vapor pressure of the pure component times its liquid-phase mole fraction. In further explanation, the vapor-phase partial pressure is equal to the total system pressure times the component mole fraction in the vapor phase. In practice, however, usage requires the... 

The difference of usage can be illustrated by considering the solvent in a particular solution. Let be its chemical potential and let x be its mole fraction. According to the usage of the present chapter its activity coefficient y and its activity a are defined by the relations... 

Additionally, in normal usage, the composition of an ionic solution is very frequently expressed in terms of molaUty ( number of moles per kg of solvent) or coneentration (moles per hter of solution), rather than molar fraction. 

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