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The process by which a pure solvent passes through a semi-permeable membrane into a solution of the same solvent is called “osmosis.” Our task is to develop a model which accounts for osmosis. We once again turn to the concept of dynamic equilibrium.
Before the solute is added and the two flasks contain only pure water, the rate of flow of water from the left to the right must be exactly the same as the rate of flow of water from the right to the left. If this were not true, the water levels would be constantly changing. So, before adding the solute, we start with dynamic equilibrium. Adding the solute disrupts this equilibrium, since the water levels start to change once the solute is added. Since we only add the solute to the flask on the right, the solvent on the left is unchanged and the flow of water from left to right does not change. To account for the net flow of water from left to right, the flow of water from right to left must decrease when we add the solute. This once again sounds familiar. The presence of the solute must inhibit the flow of water through the membrane. Either the solute particles block some of the passages in the membrane, or some of the water molecules are bound up in solvating the solute particles and therefore cannot pass through the membrane. Viewed either way, the solute slows the flow of water from right to left, so there is a net flow of water from left to right. That is why we observe osmosis.
The left flask will always contain pure water in this set-up, since solute never travels from right to left. This would suggest that the osmosis should continue until there is no water remaining in the left flask. But that is not what we observe. Instead, after a while, the net flow stops and dynamic equilibrium is re-established. This is not expected. How can the rate of flow of water from right to left ever rise to meet the rate of flow of water from left to right? The clue to the answer is found by looking at the taller column of water in the neck of the flask on the right. The water piling up in the column on the right generates an extra pressure on the water near the membrane, increasing the rate of flow from right to left. Once the pressure is high enough, the rate of flow from right to left matches the rate of flow from left to right and equilibrium is achieved. The pressure required to achieve equilibrium to counter osmosis is called the “osmotic pressure.” Experimental data show that the osmotic pressure, usually labeled as Π, is proportional to the molarity of the solute in the solution:
Π = MRT
Osmotic pressures can be quite high, several times more than the atmospheric pressure. This means that osmotic pressure can be a significant driving force in nature. For example, a biological cell wall is a semipermeable membrane, permitting the passage of water and some smaller molecules like O 2 or CO 2 , but not the passage of larger molecules like proteins. As a result, osmosis is the process by which the roots of plants extract water from the surrounding soil.
P vap =P * vap *X water
Using dynamic equilibrium arguments, explain why the vapor pressure is proportional to the mole fraction of the solvent.Notification Switch
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