Originally posted by phatty925
here are my three questions:
1) i never FULLY understood the definitions of osmotic pressure and hydrostatic pressure.
can someone explain?
If two solutions have different concentrations, there is a driving force for each to equilibrate and reach the same concentration. This can be done by either osmosis (movement of water) or diffusion (migration of solute). In the cases where diffusion is not possible (selective membrane usually), then movement of water is the method choice. As you know, fluids flow because of a pressure difference. The drive for water to move in order to dilute and/or concentrate a solution is the movement of a fluid, referred to as the osmotic pressure.
Hydrostatic pressure is as the name implies. Pressure from a stationary column of water. If you dive into a pool, the pain you feel in your ear is because of the hydrostatic pressure exerted by the water above you. (This assume there is water in the pool, because if there is no water, then the pain is attibuted to sudden stop syndrome and cement poisoning.)
If you have a U-tube with a semi-permeable membrane, that allows only water to pass, then you can study both osmotic and hydrostatic pressure. If solute is added to one side and not the other, then water will proceed from the pure solution to the solute containing solution in an effort to equilibrate concentrations. Because the pure solution remains pure, the two solutions cannot equilibrate. There is always an osmotic gradient (pressure). As the water moves, one column become shigher than the other, thus is weighs more. This excess weight, when divided by the cross-sectional area of the column, is the osmotic pressure.
2) difference between afferent and efferent?
I believe it is just the first letter, where one has an a and the other an e. I'm sorry, because I am not well versed in biology.
3) in gas chromatography, do the compounds with the lowest BP's elute first? i'm just confused b/c chromatography separates based on polarity and i'm wondering how that relates to gas chromatography....actually, is it that the most non polar compounds will boil off first b/c of no hydrogen bonding but polar compounds will boil off the slowest due to intermolecular forces?
The boiling point does not formally matter, because the compounds are all vaporized instantaneously when added to the chamber. However, the rate they migrate depends on a few factors. First, there is the affinity for the column, which will vary with each column and particle. Second, there are size and mass factors, which come into play with all gases. Smaller molecules have greater speeds in the gas phase (v proportional to sqaure root of T/m). The elution time therefore depends on both the affinity for the column (which often times depends on polarity) and molecular mass.
The boiling point also depends on molecular mass and intermolecular forces, so confusion is typical. As a general rule, lower bp molecules are lighter and have less intermolecular forces, so they elute first. However, the reason they elute first is not because of BP, it just happens to depend on the same factors as bp.
You've got to draw the line somewhere.
