Osmotic Pressure - Practice Questions & MCQ

Osmosis and Osmotic Pressure

Osmosis: It is the flow of solvent molecules from solution of low concentration to solution of higher concentration when they are separated by a semi permeable membrane(SPM), the concentration obviously being defined with respect to the solute. 

Semi permeable membrane consists of a network of submicroscopic pores or holes. The pore size is such that the smaller solvent molecules can move across the membrane while the movement of larger solute  molecules is hindered by the smaller pores of the SPM.

There are many phenomena which include the process of osmosis that we observe in daily lives. For example, raw mangoes shrivel when pickled in brine (saltwater); wilted flowers revive when placed in freshwater, blood cells collapse when suspended in saline water, etc.

Assume that only solvent molecules can pass through these semipermeable membranes. If this membrane is placed between the solvent and solution as shown in figure given below, the solvent molecules will flow through the membrane from pure solvent to the solution. This process of flow of the solvent is called osmosis.

The flow will continue till the equilibrium is attained. This flow of the solvent  molecules to the solution side across a semipermeable membrane can be stopped if some extra pressure is applied on the solution. This pressure that just stops the flow of solvent is called osmotic pressure of the solution. The osmotic pressure is represented by the letter  (Pi). 

This is illustrated in Figure given below. The osmotic pressure of a solution is the excess pressure that must be applied to a solution to prevent osmosis, i.e., to stop the passage of solvent molecules through a semipermeable membrane into the solution.

Osmotic pressure is a colligative property as it depends on the number of solute molecules and not on their identity. For dilute solutions, it has been found experimentally that osmotic pressure is proportional to the molarity, C of the solution at a given temperature T. Thus:

Here  is the osmotic pressure and R is the gas constant. The above equation can also be written as

Here V is the volume of a solution in litres containing n2 moles of solute.

If w₂ grams of solute, of molar mass, M₂ is present in the solution, then   and we can write,

Thus

Thus, knowing the quantities w₂, T,  and V we can calculate the molar mass of the solute.