Before we introduce the final colligative property, we need to present a brand-new concept. A semipermeable membrane is a thin membrane that will pass certain small molecules, however not others. A slim sheet that cellophane, for example, acts as a semipermeable membrane. Consider the device in figure (PageIndex1).

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Figure (PageIndex1): Osmosis. (a) Two options of different concentrations are placed on either side of a semipermeable membrane. (b) once osmosis occurs, solvent molecules selectively pass through the membrane from the dilute equipment to the focused solution, diluting it until the 2 concentrations space the same. The press exerted by the various height of the solution on the right is referred to as the osmotic pressure. (CC BY-SA-NC 3.0; anonymous) A semipermeable membrane separates 2 solutions having the different concentrations marked. Curiously, this situation is no stable; over there is a tendency for water molecules to relocate from the dilute side (on the left) to the focused side (on the right) till the concentrations are equalized, together in figure (PageIndex1b). This propensity is called osmosis. In osmosis, the solute stays in its initial side the the system; only solvent molecules move through the semipermeable membrane. In the end, the two sides the the system will have various volumes. Because a column of liquid exerts a pressure, there is a pressure distinction (Π) on the two sides of the system that is proportional come the height of the higher column. This pressure difference is called the osmotic pressure, i beg your pardon is a colligative property.

The osmotic press of a equipment is basic to calculate:

where (Π) is the osmotic pressure of a solution, M is the molarity that the solution, R is the appropriate gas legislation constant, and also T is the pure temperature. This equation is reminiscent of the ideal gas law we taken into consideration in chapter 6.

Exercise (PageIndex5)

What is the osmotic press of a 0.0522 M solution of C12H22O11 at 55°C?


1.40 atm

Osmotic pressure is crucial in biological systems because cell walls space semipermeable membranes. In particular, once a human being is receiving intravenous (IV) fluids, the osmotic pressure of the fluid needs come be roughly the same as blood serum to stop any negative consequences. Number (PageIndex3) mirrors three red blood cells:

A healthy red blood cell. A red blood cell that has been exposed come a reduced concentration than regular blood serum (a hypotonic solution); the cell has actually plumped up together solvent moves into the cell to dilute the solutes inside. A red blood cabinet exposed come a greater concentration than typical blood serum (hypertonic); water pipeline the red blood cell, so it collapses ~ above itself. Only once the solutions inside and outside the cell space the exact same (isotonic) will the red blood cell be able to do that is job.

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alttwo solutions are separated by a semipermeable membrane