osmotic pressure


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Osmosis is the movement of solvent molecules through a semipermiable membrane.When two solutions of the same solvent are seperated by a semipermiable membrane,solvent molecules migrate through the membrane from the solution of low concentration to the solution of high concentration.

Certain membranes allow solvent molecules to pass through them but not solute molecuels,such a membranes is called semipermiable.

Figure above depicts the operation of a semipermiable membrane.This is an experiment to demonstrate the osmosis.

A concentrated glucose solution is placed in an inverted funnel whose mouth is sealed with a  semipermeable membrane.The funnel containing the glucose solution is then placed in a beaker of pure water.As the water flows from the beaker through the membrane into the funnel,the liquid level rises in stem of the funnel.

The glucose solution will continues to rise up the funnel stem until the downward pressure exerted by the solution above the membrane eventually stops the upward flow of solvent in this case water.

In general, osmotic pressure is a colligative property of a solution equal to the pressure that when applied to the solution just stops osmosis.

So, the pressure increase needed to equalize the transfer rates is called osmotic pressure Π.

       Π   = MRT

osmotic pressure Π of a solution is related to the molar concentration of solute M.R is the gas constant & T is the absolute temperature.

Osmotic pressure plays an important role in the bilogical chemistry because the cells of human body are encased in semipermeable membrane and bathed in body fluid.The solution surrounding the cell must have an osmotic pressure equal to that within th cell.Otherwise, water would either leave the cell,dehydrating it or enter the cell and possibly burst the membrane. 

Determination of molar mass

The measurment of   Π  provides a convenient way to determine the molar mass of a compound.The osmotic pressure equation can be solved for the molar mass after molarity is expressed in terms of mass and molar mass;

  Π = MRT so   Π  = mRT/V (MM)

rearrangment gives an equation for molar mass: 

 MM = m RT/  Π   V