Where do you want to go ?

VLE Equations for a Single Condensable Species

Recall, from page 2, the equation that defines the partial pressure and apply it to each species in the gas phase.

Pi = (yi) (Ptotal)

Pj = (yj) (Ptotal)

and

Where :

Pi and Pj are the partial pressures of species i and j
yi and yj are the vapor phase mole fraction of species i and j
Ptotal is the total pressure in the system

Now we have two species, liquid and non-condensable gas, so:

yi + yj = 1

Also note that the sum of the partial pressures will equal the total pressure:

Pi + Pj = Ptotal

Lesson
2D Blog

Confused and have questions? We've got answers. With Chegg Study, you can get step-by-step solutions to your questions from an expert in the field. If you rather get 1:1 study help, try 30 minutes of free online tutoring with Chegg Tutors.

Roll your mouse over this box to close.

Join Learn Thermodynamics Advantage

Download Data Tables
Download Study Aids
Homework problem hints and answers
Get Help from Dr. B in the LT Blog
120 day membership

Click here to Log-In to your LTA account

Get it ALL for $5 US

Lesson 2D: page 4 of 11

We begin analyzing our system with a single condensable species by writing equations for the partial pressure in terms of the gas phase mole fractions.
Since we know that there are only two species in our system, we also know that the two mole fractions add up to one.
In other words, all of the molecules in the system are either blue or red, the condensable species or the non-condensable species.
Now the cool part. If we solve the equations for the partial pressures of i and j for the mole fractions and add them together and set the sum equal to one, we get an interesting result.
The sum of the partial pressures of all the species in the gas phase must be equal to the total pressure.
That shouldn’t be a big surprise. Once again, the partial pressure of a species is the portion of the total pressure that is caused by that species.
Now let’s consider what is going on at the vapor-liquid interface at equilibrium.

VLE Equations for a Single Condensable Species

Ch 2, Lesson D, Page 4 - VLE Equations for a Single Condensable Species