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Foehammer-1
03-29-2008, 02:04 PM
I posted this on both SH3 and 4 forums, hoping for some help

Hey all, i know this isn't the place to post, but.... I really hope that at least some of you didn't forget basic chemistry...

I did a lab on partial pressures, and it's due monday.

Here's the problem. We use a microtube with a liquid (water, methanol and then ethanol) and an air bubble, put that into a warm water bath, and heat it gently, recording how the volume of the bubble changes.

Then calculating partial pressure using formulas:

P air = (RT*n air) / Volume of gas

and

P vapor = 1atm - (RT*n air) / Volume of gas

Now, my results show that pressure of air increases. Which makes sense. But according to second formula, the vapor pressure goes down... Which makes no sense to me?

Can anyone please tell me if it sounds right?

I can't reach any of my friends who know this..

Help much appreciated!

Foehammer-1
03-29-2008, 02:04 PM
I posted this on both SH3 and 4 forums, hoping for some help

Hey all, i know this isn't the place to post, but.... I really hope that at least some of you didn't forget basic chemistry...

I did a lab on partial pressures, and it's due monday.

Here's the problem. We use a microtube with a liquid (water, methanol and then ethanol) and an air bubble, put that into a warm water bath, and heat it gently, recording how the volume of the bubble changes.

Then calculating partial pressure using formulas:

P air = (RT*n air) / Volume of gas

and

P vapor = 1atm - (RT*n air) / Volume of gas

Now, my results show that pressure of air increases. Which makes sense. But according to second formula, the vapor pressure goes down... Which makes no sense to me?

Can anyone please tell me if it sounds right?

I can't reach any of my friends who know this..

Help much appreciated!

DRAGONFIRE_Sr
03-30-2008, 03:37 AM
I'm just a HS droppout, but I think vapor presure is the point at which a liquid will become a vapor. as the temp goes up the liquid will gas off at a lower point.

Dont take my word on it though... as I said, I'm a dropout and never was much good at math or chemistry!!

dgrayson
03-30-2008, 04:57 AM
It seems you have several things going on there. If I read correctly you have air, water, methanol and ethanol in a sealed tube. As the temperature increases, the air pressure increases. At the same time the liquids expand from the warming. As you cannot compress a liquid, the volume of the air decreases as the hydrostatic pressure of the liquids increases. At the same time the boiling point of the liquids increase.

Because you are operating in an condition where you have a saturated vapor, the gas laws (Boyle's Law for example) do not apply to the water vapor and two alcohol vapors. Even though the temp is increasing, so is the internal pressure of the vessel. As that happens and the boiling points of the liquids increase, you would have condensation of the liquid with the lowest boiling point first. This would lower the vapor pressure, so it would seem you are correct.

I am looking at this from a refrigeration cycle standpoint as that is my frame of reference. Of course, I could be completely wrong.

Dave

Foehammer-1
03-30-2008, 08:46 AM
Alrighty, sorry for all the trouble. I didn't substitute a value into an earlier formula, then dragged the whole thing for all my observations, so everything was calculated the wrong way from that point on.

Now it makes sense, pressure of air decreases, while vapor pressure increases :P