Mass Transfer
When the liquid phase and vapour phase of a binary system obeys Raoult's and Dalton's law respectively, the relative volatility is the ratio of

Vapour pressure of component A to the total pressure
Vapour pressure of component A to the partial pressure of A
Partial pressure of component A to the total pressure
Vapour pressure of component A to that of component B

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Mass Transfer
Solvent extraction is the terminology applied to the liquid-liquid extraction, which is preferred for the separation of the components of liquids, when

One of the liquid components has very high affinity towards the solvent
One of the liquid components is heat sensitive
Extracting solvent is cheaply & abundantly available
Viscosity of liquid components is very high

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