Monday, September 6, 2010

Osmosis - a tale of dilute solutions, and semipermeable membranes.

So my first blog for this chapter is going to be focussing around osmotic pressure, and some of the details surrounding it.

So to describe osmosis, the book detailed the example where water is placed into a two part chamber, with a semi-permeable membrane seperating the two parts. If we add solute to one side such that there is a concentration difference, the water passes through the membrane till the concentrations and pressures of the two systems have come to equilibrium.
If you'd like another way to describe this, I'll explain the way I once heard it told. Take two connected chambers like before, except this time put in a sliding membrane. As we add more solute to one side, the interactings of the molecules moving about forces the piston away to the point where the forces on either side of the piston equilibrate.

There are a couple of points we should look at in relation to pressure.
Pi = cRT (which is essentially the same as p = nRT / V, since n/V = c)
We can treat a dilute solution the same way that we can treat an ideal gas, since the molecules don't interact much with each other.

Next up we have the vant Hoff relation, which in section 7.3.2 is clarified as being the pressure needed to stop osmotic flow, or as a better way of putting it, the pressure drop if the system were brought to equilibrium.

While on the topic of equilibrium, it will be worth mentioning that our cells have yet again proven their awesomeness, by being created to have the ability to fine-tune internal solute concentrations, so as to survive in a world of osmosis, especially since the book performs calculations which confirm that our cells could be destroyed quite easily by osmosis.

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