How to Calculate the Mole Fraction of a Substance in the Vapor – Part 2

Vapor pressure or equilibrium vapor pressure, an indication of a liquid’s thermodynamic tendency to evaporate, is the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases (solid/liquid) at a given temperature in a closed system.


Raoult’s law states that the partial pressure of each component of an ideal mixture of liquids is equal to the vapor pressure of the pure component (liquid/solid) multiplied by its mole fraction in the mixture.

Dalton’s law (aka Dalton’s law of partial pressures) states that the total pressure exerted is equal to the sum of the partial pressures of the individual gases in a mixture of non-reacting ideal gases.

Question:

Two liquids A & B have original vapor pressures in the ratio of 1:3 at a certain temperature. Assuming that A & B form ideal solution and the ratio of mole fractions of A & B in the vapor phase is 4:3, what is the mole fraction of B in the solution at the same temperature?

Solution:



Click/tap to enlarge the image

p = Vapor pressure, p* = Original vapor pressure, χ = Mole fraction

Messy mole fractions❓ Let Chemaficionado know in the comments below or at mychemistryhomework@gmail.com❗

References:

(1)    Colligative properties. Wikipedia. https://en.wikipedia.org/wiki/Colligative_properties.

(2)    Ncert. Chemistry : Textbook for Class XII - Part.I; National Council Of Educational Research And Training: New Delhi.

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