Trouble in understanding a few points of colligative properties

  • #1
I am having a little trouble in understanding a few points of colligative properties. Any help will be appreciated.

First, let's consider RLVP. We know that addition of a non-volatile solute decreases the vapor pressure of a pure volatile solvent. However, I am getting confused between the terms "solvent" and "solution". Here, is the vapor pressure of the solvent reduced, or that of the whole solution? As we can see, vapors of solute are not present as it is non-volatile, hence vapor pressure of the whole solution is simultaneously decreased (meaning that it is less than that of pure solvent).

Now, let's consider elevation in BP. As we add a solute to a pure volatile solvent, is the BP of the solvent increased or that of the whole solution? I went through many solved examples, and I am guessing that it is the solution whose BP is increased: BPsolution = BPpure solvent + ΔTb, where ΔTb=kb×m×i

I have a couple of questions for you:
1) In the calculations of CP, is the solute ALWAYS considered to be non-volatile? If not, then please explain how vapor pressure and BP will be affected by a volatile solute (added to a volatile solvent).
2) The solvent/solution dilemma, especially in elevation in BP and depression in FP. Does the BP of the solvent remain unchanged?
Please try to keep your answer as basic as possible, avoid complicated explanations.
 
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Answers and Replies

  • #2
mjc123
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1) Have you heard of Raoult's Law?
2) In the case of an involatile solute, the vapour is pure solvent, and what boils or freezes is pure solvent. However, the vapour/solid is not in equilibrium with pure liquid solvent, but with a solution of solute in solvent. Therefore we talk about the VP/BP/FP of the solution. Generally when we talk about the VP/BP/FP of "solvent", without further qualification, we mean the pure solvent (pure liquid in equilibrium with its vapour/solid).
 
  • #3
Yes, I am aware of Raoult's law, and the case in which we mix a volatile solute and volatile solvent. But in (1), I am specifically taking about the four colligative properties (RLVP, ELEVATION IN BP, DEPRESSION IN FP, OSMOTIC PRESSUE).
Thanks for your explanation.
 
  • #4
mjc123
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It's more complicated, and real solutions often deviate from Raoult's Law. But note that if the solute is more volatile than the solvent, the vapour pressure is increased, and the boiling point lowered. Freezing depends on whether the components are miscible in the solid; if they are not, the FP is lowered.
 

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