help out a fellow with a chemistry question

Date 05-02-2007 18:26 Edited 05-02-2007 18:32

"Well...the reason i ask is we have been given a bonus question for my general chemistry class and i was wondering if anyone in here would be able to help me with the answer to it. This question has the potential of raising our final grade a full letter grade!!!! So i need to answer the question telling what formulas or rules i used to determine the answer...so any help please guys!

Question is as follows....

I filled a glass with 40% ethanol solution and left it open on the lab bench. When I came back next morning to the lab, I found that the ethanol concentration in the glass was still 40%.
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Eric"<----quote

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Eric, a guy on another forum is asking a general chemistry question that was asked of him by his teacher...the correct answer will award him with bonus points for his class grade.....

I was thinking that the room is at a saturation point where no more can be evaporated, so the ethanol solution doesn't leave the glass and remains(relative saturation). I also thought along the lines of the solution consisted of something that evaporated at the same rate so that regardless of the quantity of solution is left the next morning the ratio was still at 40%.....

Any other ideas to help him out with this....????

By aevald

Date 05-02-2007 18:47

Ok John, here's another stab at the question. The statement was that the solution was 40% ethanol, the other 60% wasn't specified, is it possible that this other solution was a binder that doesn't seperate from the ethanol, once in solution? I am certainly not a chemist but sometimes there are "trick questions" that need to be looked at outside of the box so to speak. So in a sense as you also stated, the evaporation rate could either be the same or the liquid as a whole could be bound together so that the evaporation occured simultaneously? Just a thought. Regards, Allan

By jwright650 (*****)

Date 05-02-2007 18:50

Allen,
Good thought!

keep em coming!

By jwright650 (*****)

Date 05-02-2007 19:36

How about the temperature in the room was such that the ethanol solution's vapor pressure was equal to atmospheric pressure? (vapor pressure equilibrium)

By Lawrence (*****)

Date 05-02-2007 19:41

Alan your so close

My guess is that rather than a "binder" it is a perfect soluable mix (water)

So in effect we are talking about 80 proof vodka here.

The Vapor Pressure for ethanol is: 40 mmHg @ 13°C

My guess is that the vapor pressure for water is such that they both are reduced equally.

If you spilled alcohol and water seperatly the alcohol would evaporate before the water, but when we set down our adult beverage and pick it up sometime later we may be confident that its highly sought after properties remain undiminished.

I'm soon to be off to the lab (liquor cabinet) to prepare further experiments. >snort<

By aevald

Date 05-02-2007 19:58

Thanks for the lesson, Lawrence it makes perfect sense now. I guess I might have to head off to the cabinet too and check this phenomenon out as well. Regards, Allan

By jwright650 (*****)

Date 05-03-2007 19:40

quote by Eric--->
"Well...what lawrence said is pretty much what i think it might be also... the teacher gave me a hint today and said thinking about the hydrogen bonds in ethanol and water...so basically it has something to do with the hydrogens in each part bonding together so that they evaporate at the same rate. I think Lawrence's answer is half of it...but i need a little more, what he said though makes ALOT of sense and i believe he is very close!

John, thanks for posting on the other forum...tell Lawrence the hint about the hydrogen bonds if possible for me and see if he has any idea, or if anyone on here has any other ideas...thanks alot.

Also it deffinatly is an ethanol water solution...got that cleared up today."<---end quote by Eric

Just an update from yesterday.

By Bill M

Date 05-07-2007 17:23

Just a thought in another direction-
I remember that the alchohols are very hydroscopic. (it pulls water out of the air.)

Maybe the same way that ethanol's hydroscopic properties cause it to absorb moisture in my gas tank. ...caused it to absorb the humidity in the atmosphere in the room...replacing any evaporated material in the container, resulting in a net loss of zero.