Mass Transfer
The expression, y(1 - x)/x(1 - y) is for

Relative volatility
Selectivity
Murphree efficiency
Absorption factor

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Mass Transfer
Henry's law states that the

Partial pressure is equal to the product of the mole fraction and total pressure
Partial pressure of a component over a solution is proportional to its mole fraction in the liquid
Partial pressure of a component over a solution is proportional to the mole fraction in the vapour
Vapour pressure is equal to the product of the mole fraction and total pressure

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