Re: pinhole osmosis

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From: Paul Doherty (pauld@exploratorium.edu)
Date: Sat Apr 16 2005 - 19:55:19 PDT 

Message-Id: <223C1A72-AEEC-11D9-927F-000A95B38012@exploratorium.edu>
From: Paul Doherty <pauld@exploratorium.edu>
Subject: Re: pinhole osmosis
Date: Sat, 16 Apr 2005 19:55:19 -0700

Hi David

I treat osmotic pressure as I treat the ideal gas law.

For gasses PV = nRt

for osmotic pressure pi V = nRT where pi is osmotic
pressure.

similar equations.

Of course the ideal gas law must be modified from this simple form to
deal with denser gasses and gasses with larger diameter molecules. Yet
it is a great first approximation.

The same thing is true of osmotic pressure. The ideal form of the
equation is independent of solute concentration and solute molecular
size, But in reality these terms do modify the first approximation
equation.

Paul D

On Apr 16, 2005, at 9:18 AM, David Lauter wrote:

> Recently a few of us biology teachers at SFUSD had an interesting
> discussion about osmosis while reviewing texts.  I'd be curious to
> know how you out there explain osmosis.  In our discussion, some of us
> and some texts emphasize that the rate of osmosis is is driven by  the
> dffusion gradient of water.  The potential energy of water can be
> calculated from the concentration of total solutes (osmolarity), so
> osmosis is a colligative property,  not dependent on specific
> properties of different solutes.  Others emphasize that ions and polar
> molecules attract water thereby reducing the energy of water.  This
> causes a reduction in the activity of unbound (free) water.  My
> problem.  I think both are right but I don't like emphasizing the
> latter explanation because this doesn't explain how  osmosis is a
> colligative property.  I would think that a larger solute molecule
> would take up more space and attract more water molecules than a
> smaller solute molecule, but apparently not.  So my two questions
> are:  1)How is osmosis a colligative property given that different
> solutes must take up different amounts of space and attract different
> amounts of water molecules? and 2) How should I teach this stuff to
> biology students?
> David Lauter
> G. Washington High
> SF
>
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