When you have a inverted thistle funnel with a semi-permeable membrane around it and you place a 2M solution in it and you place the thistle funnel with solution in a container of pure solvent, why does water stop moving upwards rather than continue moving upwards? I know it has to do with osmotic pressure but I cannot understand how.
Could it be that as water moves in pressure of liquid due to gravity (hdg) increases at at 1 point the pressure is just enough to stop osmosis.

And one more question. The more concentrated a solution is the higher its osmotic pressure, right and we say that water moves from low to high osmotic pressure?

If you have a container with a semi-permeable membrane around it and pure water in it and you place this container in a concentrated solution, osmosis stops when all water moves out of the small container right?

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If we say that osmotic pressure is the pressure that must be applied to a solution to prevent osmosis, why do we use the equation of pressure PV = nRT with n being moles of solute and V = volume of solution?

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When you have a inverted thistle funnel with a semi-permeable membrane around it and you place a 2M solution in it and you place the thistle funnel with solution in a container of pure solvent, why does water stop moving upwards rather than continue moving upwards? I know it has to do with osmotic pressure but I cannot understand how.
Could it be that as water moves in pressure of liquid due to gravity (hdg) increases at at 1 point the pressure is just enough to stop osmosis.

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Why do we give osmotic pressure a negative sign in biology but a positive sign in chemistry? Could it be that in biology we refer to the solution while in chemistry w refer o the pressure which must be applied to that solution.

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Originally Posted by Roddilla

When you have a inverted thistle funnel with a semi-permeable membrane around it and you place a 2M solution in it and you place the thistle funnel with solution in a container of pure solvent, why does water stop moving upwards rather than continue moving upwards? I know it has to do with osmotic pressure but I cannot understand how.
Could it be that as water moves in pressure of liquid due to gravity (hdg) increases at at 1 point the pressure is just enough to stop osmosis.

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Let's put it like that. When you just put the inverted funnel (like in the picture of your previous thread), at the level of the membrane, you have osmotic pressure + liquid pressure.

These are enough to push the weight of the solution column in the funnel up. Then, there is a point where the weight of the solution column equals the pressure of the liquid + osmotic pressure.

And strictly speaking, I would say that osmosis does not stop. Particles in the fluid state are in constant motion, and hence, the solvent is constantly moving through the membrane in both directions.

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And one more question. The more concentrated a solution is the higher its osmotic pressure, right and we say that water moves from low to high osmotic pressure?

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Yes, since the region of high osmotic pressure is the region where there is the least number of water molecules (also meaning the most number of solute).

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If you have a container with a semi-permeable membrane around it and pure water in it and you place this container in a concentrated solution, osmosis stops when all water moves out of the small container right?

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That's exactly the state that a biologist would call 'crenation' in a cell.

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If we say that osmotic pressure is the pressure that must be applied to a solution to prevent osmosis, why do we use the equation of pressure PV = nRT with n being moles of solute and V = volume of solution?

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I don't really understand your question here. I have the feeling you're asking why to get the energy that a certain amount of mass represent, while the answer is simply that this equation works. :confused:

If you want the full derivation, wiki has an article for it, though it might be too complicated of a derivation:

Osmotic pressure - Wikipedia, the free encyclopedia

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Why do we give osmotic pressure a negative sign in biology but a positive sign in chemistry? Could it be that in biology we refer to the solution while in chemistry w refer o the pressure which must be applied to that solution.

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I wasn't aware that pressure can be negative too :confused: (without any comparison between two pressures acting in opposite directions that is)

What do you mean by liquid pressure exactly?

My question is about the equation for osmotic pressure. If we say that the osmotic pressure is pressure that must be applied so that osmosis stops, then the PV = nRT should apply for the object causing that pressure and not the solution since this is not causing pressure?

It's the pressure of the liquid, whatever that is, depending on the context. If it's water, the liquid is water, if it's another solvent, that solvent is the liquid and that pressure is given by:

So the osmotic pressure is then created because of solvent and pressure created hdg

And could you please explain this once more: we say that osmotic pressure is the pressure created to stop water for example from net osmosis. So why don't we say that PV = nRT for the object creating pressure since the water is not creating the pressure?

What I mean is that why we include n as the number of moles of the salt when in fact it is not the salt which is creating the pressure...

No, is the formula to get the pressure at any depth from the surface, and I used that to explain why the level of solution in the funnel rises.

That, plus osmotic pressure will support the weight of the solution column in the funnel.

The solvent alone, will create P = hdg
The solvent and the solution create the osmotic pressure, PV = nRT
Together, both pressures support the weight of the solution.

We take n as being the number of moles of the solvent, because it is the solvent which determines the concentration of the solution, and hence, the magnitude of the pressure. It's first because of the solvent, THEN water will move.

What do you mean by semi-permeability of the cell

You can try Google and I'm sure you'll get a lot of links :)

Briefly, a semi permeable membrane is one which allows only certain molecules to pass through it, generally, smaller molecules. Using such a membrane, you can 'filter' salt water and get fresh water.