DATBootcamp question (didn't find answer for it anywhere)

[nemoaryan]

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Hey guys,

I am confused by the following question:
As pressure decreases at a constant temperature for the phase diagram below (it is liquid phase digram), which of the following can occur?
I. Crystallization
II. Sublimation
III. Fusion
IV. Vaporization

The answer is (I, II, IV).
what I don't get is why vaporization happens? I know that when we increase pressure (decrease volume), contestation happens as gas molecules are trapped and pressure is high so they go to liquid phase. But in the question above, it says pressure decreases while temperature is constant..so how can it vaporize? PLEASE help! Thanks!

 

[shonster460]

Well to answer this question correctly you just need to follow the phase diagram. Start at a point in the liquid part of the diagram and then decrease pressure at constant temperature (move straight down vertically). You enter the gas phase, which is vaporization.

To answer your question. How does this physically occur? Well vaporization is a result of molecules at the surface of the liquid going into the gas phase. To accomplish this, there needs to be a high amount of kinetic energy for the molecule to escape out of the surface. This is why vaporization is faster when the temperature is higher (think of boiling water). Now pressure is also important in vaporization. If the pressure is high, there is a force pushing down on the liquid, that prevents molecules from escaping. But if the pressure is lower, then the molecules can escape more easily. This is why vaporization can occur when the pressure is decreased.

I hope that answers your question!

 

[nemoaryan]

thank you sooo much!

 

[nemoaryan]

I have one more question.though not related to this one. I simply can't understand this problem or how to solve it!
What volume of HCl was added to if 20 mL of 1 M NaOH is titrated with 1 M HCl to produce a pH=2? (correct answer is 20.4 mL of acid added).

I know that it takes 20mL of NaOH to neutralize 20mL of HCl…but then i have no idea what to do from here. Thank you mucho in advance!!!

 

[TheEpicFruitCake]

Just to make sure, you crystallize because things become less soluble at lower pressures correct? aka becoming super saturated and will form precipitates right?

For your question ^^
You know that at pH 7 you will have 40mL of solution because as you stated 20mL of NaOH and 20mL of HCl neutralizes the solution. This means you have water. Na and Cl are spectators so yeah... we don't care about them anymore

From here you need to get from a pH of 7 to pH of 2 correct?
So pH of 2 = -log[H+]. Solve for [H+] = 1*10^-2M.
So you know you when you add HCl you will simultaneously be adding volume to your solution. Therefore to get your answer in molarity which is based on volume you need this:
1*10^-2M OR 0.01M (same thing) = x/(0.04L + x).
Chad says you can't do the quadratic formula on the DAT so assume x is always negligable.
revised formula: 0.01M = x/0.04L
x = 0.0004L or 0.4mL of HCl (since it disassociates completely)