[rleung]

Hi,

I just want to make sure one thing.  If you use Raoult's Law to calculate the vapor pressure of the solvent in an ethanol solution (partial pressure of solvent = i * mol fraction of solvent * pressure of pure solvent), you use i=1, right?  I am pretty sure, but it has been a while, so I just want to make sure.  I know that when ions dissociate, you change "i" accordingly (for example, if you dissolve KCl, you have to use i=2), but in this case, since this is an acid-base type problem, the dissociated H+ does not count, right?

Also, would it also be i=1 if you were trying to calculate the partial pressure of the solvent in a HCl aqueous solution?  Thanks.

Ryan

[Borek]

If there is any dissociation you must change i accordingly, deosn't matter if is it acid, base, or salt. Things can get tricky when the dissociation is not 100% (as in the case of weak acids) as i will take real, non-integer value.

[rleung]

Hi,

I have another problem with vapor pressure.  At 22 degrees C, the vapor pressure of water is 19.8 torr.  A 4-L container at 22 degrees C is saturated with water vapor (the relative humidity is 100%) and then compressed to half its volume.  What is the pressure of the water vapor after compression?

Originally, I wanted to use P1V1=P2V2 to solve the problem.  With this, I get...
(19.8 torr)(4-L) = P2 * 2-L

But when I look at the answer I wrote down long ago (this is an old problem on a practice test from long ago), I have here written that the vapor pressure does not change since the temperature is kept constant.  I do not entirely get this..if the container is saturated with water vapor, shouldn't halving the volume have some effect on the pressure?  Thanks.

Ryan

[Borek]

condensation

[rleung]

So what you're saying is that if I want to find the osmotic pressure/bp elevation/fp depression/vapor pressure depression of an aqueous solution of either CH3OH or HCl, I have to use to use i=2??? (so for example, to find vapor pressure depression, ?P = 2*Xsolute * Psolvent)

If yes, I am confused on a problem I have.  It asks me to arrange the following aqueous solutions in order of increasing vapor pressure:

0.35 m ethanol (note: molality)
0.50 m sugar
0.20 m KBr
0.20 m Na2SO4

Originally, I had the following answer: Na2SO4 < sugar < KBr < ethanol, which I do not entirely understand if what you say is true.  Assuming the kg of solvent is the same for everything (I assumed 1-kg), I will have the following MOLAR quantities:

0.35 mol ethanol --> 0.35 mol CH3CH2O- + 0.35 mol H+ (total: 0.70 mol)
0.50 mol sugar ---> I don't know how much to dissociate since I don't know the sugar (presumable total: 0.5 mol)
0.20 mol KBr ---> 0.20 mol K+  + 0.20 mol Br- (total: 0.40 mol)
0.20 mol Na2SO4 ---> 0.40 mol Na+  +  0.20 mol SO4 2- (total: 0.60 mol)

Now, since the formula for finding the change in vapor pressure is ?P = i*Xsolute * Psolvent, I can assume that the larger the molar concentration of solutes, the larger the vapor pressure lowering will be.  With that in mind, the order of deviation of vapor pressure would be as follows: KBr < sugar < Na2SO4 < ethanol.

Since the question asks us to show the INCREASING vapor pressure, and vapor pressure DECREASES with increasing molar concentration, the answer should be the reverse: ethanol <Na2SO4 < sugar < KBr, which does NOT agree with the answer I had written last year (Na2SO4 < sugar < KBr < ethanol).  NOTE: The answer I wrote down could have been wrong.....I don't recall if the teacher posted solutions to the problem).  Thanks so much.

Ryan

[rleung]

Yes, the excess water vapor will condense so that the amount of water vapor needed to saturate the container will be present.  Shouldn't that DECREASE the vapor pressure tho (since there are less moles of water present in the vapor stage)??

Ryan

[Borek]

So what you're saying is that if I want to find the osmotic pressure/bp elevation/fp depression/vapor pressure depression of an aqueous solution of either CH3OH or HCl, I have to use to use i=2???

No. 2 in the case of HCl, as it is competely dissociated (well... almost completely. Or completely enough ). In the case of acetic acid you must calculate degree of dissociation first - for example if you have 5% dissociation i=0.95+2*0.05 (you must calculate how many molecules/ions were added to the solution - there is 95% of acid left undissociated, plus 5% H⁺ and another 5% of CH₃COO^-).

0.35 mol ethanol --> 0.35 mol CH3CH2O- + 0.35 mol H+ (total: 0.70 mol)

No. Ethanol is very weak acid, ypou may assume it didn't dissociate.

0.50 mol sugar ---> I don't know how much to dissociate since I don't know the sugar (presumable total: 0.5 mol)

Generally speaking sugars doesn't dissociate at all.

0.20 mol KBr ---> 0.20 mol K+  + 0.20 mol Br- (total: 0.40 mol)

Correct.

0.20 mol Na2SO4 ---> 0.40 mol Na+  +  0.20 mol SO4 2- (total: 0.60 mol)

Correct.

the answer I had written last year (Na2SO4 < sugar < KBr < ethanol)

Correct. Compare with the corrected numbers from above.

[rleung]

Hi,

I posted this from earlier, but I think the moderator keeps merging this post with my other post on vapor pressure, so it keeps on getting lost.  At 22 degrees C, the vapor pressure of water is 19.8 torr.  A 4-L container at 22 degrees C is saturated with water vapor (the relative humidity is 100%) and then compressed to half its volume.  What is the pressure of the water vapor after compression?

Originally, I wanted to use P1V1=P2V2 to solve the problem.  With this, I get...
(19.8 torr)(4-L) = P2 * 2-L

Since halving the volume will lead to condensation, is this the right approach to solving this problem?  Thanks.

Ryan

[plu]

Vapour pressure is a constant value that is independent of volume.  It is dependent only on temperature.

[Donaldson Tan]

Vapour pressure is a constant value that is independent of volume.  It is dependent only on temperature.

Google for Antoinne's Equation

[rleung]

Ah, I see!  It would help to remember some key chemistry facts from gen chem.  Thanks SO much!

Ryan