Mass Transfer
If a two phase system is in physical equilibrium; then it means that, the

Temperature of the liquid phase is equal to that of the vapor phase
All of these
Escaping tendency of each component from the liquid phase to the vapor phase is exactly equal to that from vapor phase to liquid phase
Total pressure throughout the liquid phase is equal to that throughout the vapor phase

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Mass Transfer
Fenske's equation for determining the minimum number of theoretical stages in distillation column holds good, when the

Mixture (to be separated) shows positive deviation from ideality
Multicomponent distillation is involved
Mixture (to be separated) shows negative deviation from ideality
Relative volatility is reasonably constant

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