# VLE for a Single Condensable Species

Non-Condensable species j, RED
Measures Vapor Phase Mole Fraction, yj
Condensable species i , BLUE
Measures Total Pressure,
Ptotal
Measures Vapor Phase Mole Fraction, yi
Measures temperature,
T
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### Ch 2, Lesson D, Page 3 - VLE for a Single Condensable Species

• Why did we choose to let air into our system ?  Well, air is pretty common stuff, but it is also special because it is a supercritical fluid at common system temperatures and pressures.  As a result, it does not condense.  All of the air remains in the gas phase.  You don’t see any red dots in the liquid phase, right ?
• Now, in fact some of the air DISSOLVES into liquid water or we wouldn’t have any fish in our lakes and oceans.  But only a small amount of nitrogen and oxygen dissolve in water and this phenomenon is beyond the scope of Thermo-CD.
• Alright, now let’s consider our can again, with mostly water, but some air inside.
• It isn’t quite as easy to measure mole fractions as it is to measure temperature and pressure.  Nonetheless, for purposes of this “thought experiment,” we will imagine that such instruments are available to us.
• Here we use red circles to represent the non-condensable species, air, and blue circles to represent the condensable species, water.
• Y sub i is the gas phase mole fraction of the condensable species and y sub j is the gas phase mole fraction of the non-condensable species.
• Now that we have the system laid-out and we have described all of the variables that we can measure, let’s see what equations we can write to show how the variables that describe our system are related to each other.