ASDIC

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can someone explain me the conditions and reasons for solutions that deviate from Raoult's law...both positive and negative deviations


thanks
 

JDAD

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As far as i know, you just have to know the two versions of the law.

also know that intermolecular forces play a role in vapor pressure.
I if the solute solvent bond formation is exothermic, that means the bond formed between the two different compounds are stronger than that of each pure compound. This results in a decrease in vapor pressure.

It is opposite for endothermic solute-solvent bond formation.

Thats all you really need to know. For the MCAT that is.

If this is for Pchem, go read the CRC and then derive seven different formulas all filled will calculus and crazy latin symbols.

Good luck.
 

fun8stuff

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Originally posted by thekegalman
can someone explain me the conditions and reasons for solutions that deviate from Raoult's law...both positive and negative deviations


thanks

It is funny that you mention this. I just got done reading this in EK 5 mins ago. Do you have EK? They explained it fairly well in the general chemistry lecture 5. Their take on it was that you didn't need to know the deviations real indepth.
 

ASDIC

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well our TPR instructor went in real depth and said that it was important on the MCAT.

So she talked about the deviations based on a non-volatile solute in a non-volatile solvent, volatile solute in a non-volatile solvent, a non-volatile solute in a volatile solvent and in cases where both the solute and solvent are volatile.

So...i shudnt study the deviations then.
 
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JDAD

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Originally posted by thekegalman
well our TPR instructor went in real depth and said that it was important on the MCAT.

So she talked about the deviations based on a non-volatile solute in a non-volatile solvent, volatile solute in a non-volatile solvent, a non-volatile solute in a volatile solvent and in cases where both the solute and solvent are volatile.

So...i shudnt study the deviations then.

You answered your own question. Just use the law the way it was meant to be used.

Non-volatile solute in a non-volatile solvent will have zero vapor pressure.

Volatile solute in a non-volatile solvent will cause an increase in vapor pressure, but the vapor pressure will be solely due to the addition of solute. It would have to be pretty concentrated for this to make a differrence.

non-volatile solute in a volatile solvent causes a decrease in vapor pressure. The mole fraction of the solvent has decresed. This occurs because the non-volatile solute molecules compete for surface area and take up space.

volatile solute in volatile solvent is tricky. You have to use the law. P = XaPa plus XbPb. If the solute has a greater vapor pressure than the solvent, total vapor pressure will increase. If the solute has a lower vapor pressure, the total pressure will decrease.

Thats all you need to know. But make sure you know what will happen to vapor pressure if the rxn is exo/endothermic (i listed these in the first post)

That should do it.
 
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