General CHemistry
11.Solutions
Raoult's Law with A Non-Volatile Solute and a Volatile Solvent
Samin Nosrat, famed chef and author of Salt, Fat, Acid, Heat, notes in her Cacio e Pepe recipe that you should "season [your pasta water] generously with salt until it tastes like the summer sea." Imagine Samin is on vacation to Rocky Mountain National Park (typical barometric pressure is 753 mmHg) and wishes to make herself some Cacio e Pepe after a long day of hiking.
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a) If Samin adds 60 g of table salt (NaCl) to 2 quarts (1.89 L) of boiling pure water (99.8 deg C at this atmospheric pressure), what vapor pressure is predicted according to Raoult's Law?
b) Is it expected that the water will continue boiling as the salt dissolves?
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c) What needs to occur for the water to re-establish boiling?
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d) What is the boiling point of the mixture? What does your answer for (c) mean for the pasta-cooking process?
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e) Why would adding salt to pasta water be especially useful at high altitude?
Raoult's Law with Two Volatiles
Raoult's Law with two or more volatiles assumes ideal behavior, meaning that the intermolecular forces that exist between molecules of different identity in the solution are of equal strength than those that exist between molecules in each pure component.
The plots above show three different mixtures of real solvents. Answer the following questions:
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a) Which mixture behaves the most ideally?
b) Using the Lewis diagrams of the two components of each mixture, give the intermolecular forces that should exist between two molecules of the same identity, and then between the two components of the mixture.
c) For each mixture, indicate whether the IMFs that exist between the two components of the mixture are stronger, weaker, or roughly the same as the IMFs that exist between molecules of the pure solvents.
d) Using your answer in (c), explain the why the vapor pressure curve deviates from linearity in plots (b) and (c).