Bubble Point: The bubble point of a system at pressure P and whose liquid composition is given by mole fraction xi is that temperature at which there is onset of vaporization. Mathematically it is the temperature such that

 

 

For a binary mixture, this equation can be rearranged to yield

 

 

Example 6.2: Calculate the composition of a liquid of an ideal binary mixture of benzene and toluene at 760 mm Hg at a bubble point of 90 C. The following data apply:

Vapor pressure of benzene = 1.02103 mm Hg

Vapor pressure of toluene = 225.1 mm Hg

 

Solution: At boiling point vapor pressure is equal to the total pressure. Using this definition, boiling point of benzene and toluene can be found to be 80.1 C, and 130.13 C, respectively. The bubble point of a mixture can be found from Equation 6.7

 

 

x can be found by using Equation 6.8

 

 

The results are summarized below:

 

 

t/C

 

P1/

(mm Hg)

 

P2/

(mm Hg)

 

K1

 

K2

 

x

 

90

 

1.02103

 

225.1

 

1.34

 

0.296

 

0.672