A bottle of pure n-heptane is accidentally poured into a drum of pure toluene in a laboratory. One of the laboratory assistants suggests that since heptane boils at a lower temperature than toluene, the following purification procedure can be used:
Pour the mixture (2 mol% n-heptane) into a simple still pot. Boil the mixture at 1 atm and condense the vapors until all heptane is boiled away. Obtain the pure toluene from the residue.
You, a chemical engineer with knowledge of vapor–liquid equilibrium, immediately realize that such a purification method will not work. 

(a) Indicate this by a curve showing the composition of the material remaining in the still-pot after various quantities of the liquid have been distilled. What is the composition of the residue after 50 wt% of the original material has been distilled? What is the composition of the cumulative distillate? 

(b) When one-half of the heptane has been distilled, what is the composition of the cumulative distillate and the residue? What weight %t of the original material has been distilled? Equilibrium data at 1 atm [Ind. Eng. Chem., 42, 2912 (1949)] are:

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Liquid 0.025 0.062 0.129 0.185 0.235 0.250 0.286 0.354 0.412 Mole Fraction n-Heptane Liquid 0.448 0.455 0.497 0.568 0.580 0.692 0.843 0.950 0.975 Vapor 0.048 0.107 0.205 0.275 0.333 0.349 0.396 0.454 0.504 Vapor 0.541 0.540 0.577 0.637 0.647 0.742 0.864 0.948 0.976