Water and acetic acid do not form an azeotrope at 760 mmHg. The normal boiling point of acetic acid is 118.5°C. Therefore, at 118.5°C and 760 mmHg, the mixture will exhibit only vapor behavior across the composition range. The following equilibrium constants have been fitted to represent the vapor-phase behavior:

(a) Let compound A be acetic acid and B be water. Calculate the true mole fractions of all the species from y_A = 0.05 to y_A = 0.95. At what apparent mole fraction does each specie show a maximum true mole fraction? What is the relation of this apparent mole fraction with the compound’s stoichiometry?

(b) Plot the fugacity coefficient of acetic acid and water as a function of acetic acid mole fraction. What is the physical interpretation of the rapid change of the acetic acid fugacity coefficient in the dilute region, if the water fugacity coefficient doesn’t show such a dramatic trend in its dilute region?

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Dimer, -log₁0K, where K in (mmHg)¯¹ Acetic acid Water Acetic acid/water complex | Same as water 10.108-3018/T(K) 6.881-808.2/T(K) Trimer, -log₁0K, where K in (mmHg)-² 18.63-4960/T(K)