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Q & A: Baloney with solutions

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Most recent answer: 03/19/2011
Yahoo's physics Q&A site is quite good for a very open site, but as might be expected it's not as reliable as one would like. Here's an example of an error, although not an extreme error. Question: If you have a solution of 0.52A+0.48B (by mole fraction), and the vapor pressures of A and B are 114.9mmHg & 238.3 mmHg respectively, what's the total vapor pressure?
- Mike W (age 61)

The answer given was "0,52*114,9 + 0,48*238,3 = 174,1 mmHg" (the commas were Euro-style). This assume a completely ideal solution of the components A and B. Solutions are rarely ideal to nearly the precision given in the answer. The reason is, to put it in crude terms, that A and B don't stick to each other exactly as well as each sticks to itself. As a result, the vapor pressure of the solution is almost always noticeably higher or lower than the simple weighted average.

There's a nice discussion of the rule that they used and the deviations from it on 

One very familiar example of a solution with strong deviations from the simple rule is the water-ethanol combination, described here: :

I'm being a little picky here. The reason is that in thermodynamics there are some amazing exact relations, so it's important not to get various rules-of-thumb mixed up with the exact laws. One of the surprising exact laws in this specific area (vapor pressure of solutions) is discussed on another of our answers.

Mike W.

(published on 01/19/2011)

Follow-Up #1: comma vs.period

"The answer given was "0,52*114,9 + 0,48*238,3 = 174,1 mmHg" (the commas were British-style)." The British don't use a comma.
- Dan
fixed, thanks

(published on 03/19/2011)

Follow-up on this answer.