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# chem 2445-midterm example%2b answers (2) - Name CHEM 245...

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Name: CHEM 245- Midterm-like questions NO CALCULATORS ALLOWED IN THE EXAM 1. A compound boils at 400 ° F at atmospheric pressure. Since it decomposes easily at elevated temperatures, you need to bring the bp down to 60 ° C. In the lab, you have a vacuum pump that allows you to drop the pressure to 200 mm Hg. (a) What is the bp now? (5p) 150 C or 300 F. (b) At what pressure is the bp 60 ° C? (5p) ~4 mmHg (c) How much water per gram of compound do you need to add (under the conditions of point a) to make it boil at 60 ° C? P water @60°C = 150 mm Hg and molecular wt of compound = 100 and it is immiscible with water. 7.5p (d) How much water do you need if the pump is not available? 7.5p. (a), (b), connect the respective points on the graph. Pay attention to the F vs C. (c) In point a, the pressure above the liquid is already down to 200 mm Hg, by means of the vacuum pump, but the boiling point is still high, 150 C, and you want to drop it down to 60 C, by adding water/steam. Note that in this case the compound boils at 200 mm Hg, not 760 mm Hg. 1 150 C or 300 F 3-8 mm Hg

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Thus, the pressure above the liquid is 200, not 760 as in the original formula. Thus, the amount of water is given by: Wt water = (18 * P water )/[Mwt*( 200 -P water )]= (18*150)/[100*(200-150)] = 0.540 g water/g (d) The pump is not available, means atmospheric pressure. Yet the compound should still boil at 60 C. ( i.e. P water
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