phatty925

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here are my three questions:

1) i never FULLY understood the definitions of osmotic pressure and hydrostatic pressure.
can someone explain?

2) difference between afferent and efferent?

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?
 

Mudd

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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.
 

Mr. Z

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2) When you think of afferent and efferent do so from the position of the CNS. Afferent means towards the CNS, efferent means away from the CNS.

3) I think for the mcat they want to think of chromatography as separating compounds based on boiling point. Lowest boiling points eluting first, highest eluting last. Though, in reality this is not always the case. Polarity does play a role, but it depends on the liquid phase that's chosen.


don't know what to say about #1 other than it is directly proportional to molarity.


I do have a question though... would ion pairing decrease the osmotic pressure of a solution?
 
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lola

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my current knowledge:


1.
hydrostatic pressure is a pressure outward on a blood vessel due to e.g. the pumping of the heart. osmotic pressure is the pressure inward on a blood vessel due to solute concentration in the vessel. if the concentration is high, water pushes in toward the vessel.


2.
afferent -- going towards: like sensory nerves are going toward your spinal cord, brain

efferent -- going away: like motor nerves are going away from your spinal cord, brain

i'm pretty certain this is a general definition for afferent/efferent and can be used with other examples like blood vessels.
 

Mudd

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Originally posted by Mr. Z
I do have a question though... would ion pairing decrease the osmotic pressure of a solution?
Definitely! It reduces the i in Posmotic = MiRT and it changes the solvation.

PS: Z, you are going to nail this MCAT like no other. I'm guessing you pull about 37. On a good day, with the right proctor and good karma, you're probably a 41.
 

Focus

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For the MCAT, the questions regarding pressure would most likely deal with blood vessels in regards to hydrostatic pressure and osmotic pressure.

The main thing you need to know is:

The force behind fluid trying to escape the blood vessel is called hyrostatic pressure and the force pushing the fluid back into the vessel is called colloid, or oncotic pressure (which is caused by those proteins called Albumins within your blood vessel)

These two forces counter balances and adjusts accordingly in the human body.

- Focus
 

Mr. Z

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Mudd, thanks for the vote of confidence. I only hope that you are right. I'm guessing with the right proctor and a decent sized bribe i may very well hit 37!
 

Mudd

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Originally posted by Mr. Z
Mudd, thanks for the vote of confidence. I only hope that you are right. I'm guessing with the right proctor and a decent sized bribe i may very well hit 37!
Keep in mind, when graphing MCAT score as a function of size of bribe, it is exponential, not linear. Bring the big checkbook.

Seriously, the way you present things is systematic and logical, not to mention right on the mark. People who think the way you do often nail this test. BTW, some proctors may accept things other than money.
 

Mudd

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Originally posted by Mr. Z
I'll only consider going there if the proctor is female;) You've got to draw the line somewhere.
Very true... not to mention that the ability to distinguish male from female might prove beneficial in medicine. They'll be impressed with you at interviews.
 
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