Xref: utzoo sci.bio:3783 sci.chem:2354 sci.physics:15169 sci.misc:4520
Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!mcsun!hp4nl!ruuinf!ruunsa!chooft
From: chooft@ruunsa.fys.ruu.nl (Rob Hooft)
Newsgroups: sci.bio,sci.chem,sci.physics,sci.misc
Subject: Re: Osmosis - the cause at the molecular level.
Message-ID: <1722@ruunsa.fys.ruu.nl>
Date: 1 Nov 90 08:04:16 GMT
References: <1990Oct28.115303.7221@newcastle.ac.uk> <4396@pkmab.se>
Followup-To: sci.bio
Organization: University of Utrecht, Dept. of Physics
Lines: 34

In <4396@pkmab.se> ske@pkmab.se (Kristoffer Eriksson) writes:

>If the water remaind and equal levels, gravity would also act equally on
>both sides, and would therefore not stop the sides from differentiating.

>I suppose that if one of the sides would rise considerably above the
>other side, it could be compressed enough by gravity to make the number
>of water molecules to a given surface area on both sides equal slightly
>before the concentrations become equal. (The water pressures would become
>equal without the solution concentrations being equal.)

This is the only way of stopping the process if we only add salt to one of
the two compartments. The osmotic value is also named osmotic pressure, and
if you calculate some values, say for a 0.1 M NaCl solution, 

(0.2 Moles/liter so, since

                            number of moles.R.Temperature in K
       osmotic pressure =  -----------------------------------
                                     Volume in M3

= 200 mole/m3 * 8.31441 J/K/mole * 298 K = 500000 Pa = 5 Bar ) 

then you can realize that the water in that compartment would have to rise 50
meters to obtain equilibrium (if dilution is not taken into account). In fact
this method is being used to measure molar masses of compounds. A rise of 1
cm in the compartment means 0.00004 mole/liter concentration. So the
procedure is simple: weigh x grams, put it into the compartment of y ml, if
it rises z cm it is p moles, such that the molar mass is x/p gram/mole.

I hope this makes things somewhat more clear.
-- 
Rob Hooft, Chemistry department University of Utrecht.
hooft@hutruu54.bitnet hooft@chem.ruu.nl chooft@fys.ruu.nl