TBR Section 7 Passage 6 Question 40

[ravupadh]

Hey there... I'm a little confused why D is the right choice in stead of C. Shouldn't the total vapor pressure be greater above the 10th aliquout since it has more of the 2-propanol in it? And shouldn't Aliqout 5 have the greater amount of acetophenone since it's less distilled?

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[BerkReviewTeach]

Hey there... I'm a little confused why D is the right choice in stead of C. Shouldn't the total vapor pressure be greater above the 10th aliquout since it has more of the 2-propanol in it? And shouldn't Aliqout 5 have the greater amount of acetophenone since it's less distilled?

Each subsequent aliquot has a slightly higher percentage of acetophenone and smaller percetage of 2-propanol, because the solution in the distillation flask loses more 2-propanol at first and slowly builds a higher mole fractione of acetophenone.

For instance, if the initial solution is 50-50, then the vapor at first is 80-20. As the solution loses the first aliquot (the 80-20 one), the remaining solution becomes 49-51 (or so). This means that the second aliquot will be about 78-22, leaving behind a 48-52 solution. This keeps on happening so that the 10th aliquot will be richer in acetophenone and poorer in 2-propanol than the 5th aliquot.

So the 10th aliquot will be less volatile (because it's richer in acetophenone) than the 5th aliquot and it will have more acetophenone (choice D).