Mass Transfer
Lewis number, which is important in problems involving simultaneous heat and mass transfer, is the ratio of

Thermal diffusivity to mass diffusivity
Mass diffusivity to momentum diffusivity
Momentum diffusivity to thermal diffusivity
None of these

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

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