' cij where, for a first-order approximation, ky can be set to equal to zero.
On the other hand, although Eq. (39) can be used for calculating Kn it seems somewhat sophisticated since sometimes <j)\ isn't easily derived. So one wishes to expand a simple rigorous formulation with enough accuracy. In this case, an important assumption is adopted that at middle pressure the vapor mixture can be regarded as ideal solution, but not as ideal gas. It indicates that yjV =1 in Eq. (18), which is reasonable in most cases, especially for nonpolar gases. The reason is that the long-rang forces between the gas molecules prevail and they are much weaker than in the liquid mixture. The assumption of ideal solution is virtually different from the assumption of ideal gas in that in the latter the ideal gas law is tenable.
According to the ideal gas law, not only ytv -1, but also (j>l = 1. However, for ideal solution either for gas phase or for liquid phase, just yiV =1 or yjL = 1. Thus, we can deduce another equation expressing Kt on the basis of Eqs. (18) and (32):
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