Gibbs-Dalton law

In order to evaluate the limit in Eq. (7-161) we must use the Gibbs-Dalton law which states that... 

Equation (7-162) implies that p, approaches zero as p approaches zero and thus the limits in Eqs. (7-154) and (7-157) may be taken as p approaches zero. The Gibbs-Dalton law is an empirical law that cannot be obtained from the general formalism of thermodynamics it is an additional piece of information. Equation (7-162) follows from Dalton s law and the fact that real-gas mixtures behave like ideal-gas mixtures in the zero-pressure limit. Dalton s law states that, for an ideal-gas mixture, the following equations hold ... 

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