# Partial Pressure & Vapor Pressure

Pi = Ptotal = P*

Since our system contains a pure substance at equilibrium, the vapor pressure of species i, P*(Tsat), equal to the total pressure and therefore :

Pi = (yi) (Ptotal) = Ptotal

For a pure substance there is only one species and, therefore, the vapor mole fraction is one (yi = 1). So, the partial pressure of species i is equal to the total pressure :

= the partial pressure of species i

= the vapor phase mole fraction of species i

= the total pressure

## Partial Pressure

:The FRACTION of the total pressure in a gas phase due to the presence of one particular substance.

In an ideal gas mixture :

Ptotal V = ntotal R T

Pi V = ni R T

Pi = (yi) (Ptotal)

Where :

Pi

yi

Ptotal

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