OSMOSIS AND OSMOTIC PRESSURE

Osmosis:   

When a semi-permeable membrane is placed between a solution and a solvent, it is observed that solvent molecules enter the solution and the volume of solution increases. It is also observed that if two solutions of unequal concentrations are separated by a semipermeable membrane, the solvent molecules from a solution of lower concentration move towards a solution of higher concentration. This phenomenon was first observed by Abbe Nollet (1748) and termed as Osmosis (reek, osmos = to push). Osmosis is defined as the spontaneous flow of solvent molecules through semipermeable membrane from a pure solvent to a solution or from a dilute to a concentrated solution.  

The phenomenon of osmosis can be demonstrated by the following experiment  

Two eggs of same size are taken and their outer hard shell is removed by dissolving in dilute hydrochloric acid. One of the eggs is placed in distilled water and the other in saturated salt solution.  

After sufficient time, it is noticed that the egg placed in water swells up and that placed in salt solution shrinks. In the first case water enters the concentrated egg fluid while in the second case water comes out of the egg as salt solution is more concentrated thatn egg fluid.  

Difference between osmosis and diffusion  

The two processes, diffusion and osmosis, can be distinguished in terms of the following aspects:  

(i) In diffusion, solute as well as solvent molecules flow in opposite directions while in osmosis the flow of solvent molecules occurs in one direction only.

 2065_Hydrostatic pressure determination.JPG  

(ii) For osmosis, a semipermeable membrane is required while for diffusion it is not required.  

Osmotic pressure  

A porous pot is taken and a semipermeable membrane of copper ferrocyanide is deposited in its walls. It is fitted with a long glass with the help of a rubber stopper. It is filled with concentrated aqueous sugar solution and placed is distilled water. Osmosis occurs and the level of the solution in glass tube rises over a period of time. After a few days, the level becomes stationery. At this equilibrium state the hydrostatic pressure of the liquid column exactly balances the tendency which enables the water molecules to pass through semipermeable membrane.  

The hydrostatic pressure developed as a result of osmosis is a measure of osmotic pressure of the solution. Osmotic pressure is also defined as the hydrostatic pressure built up on the solution which just stops the osmosis.  

Osmosis pressure = hydrostatic pressure  

     π = hdg  

where h = increase in level in the tube of unit cross section, d = density of solution and g = acceleration due to gravity.  

Actually, this will not be an exact measure of osmotic pressure of the solution originally taken because sufficient dilution has taken place with time.  

Alternative definition:  

An apparatus as shown in Fig. 5.10 consists of water-tight chamber which is divided into two halves by a semipermeable membrane and fitted with solution and the other compartment is filled with pure solvent. Water (solvent) tries to flow into the solution side. To check this tendency a certain pressure shall have to be applied by the piston. This external pressure is this a measure of osmotic pressure of the solution. The external pressure which must be applied on the solution in order to stop the flow of the solvent into the solution through semipermeable membrane is equal to osmotic pressure.

1909_semipermeable membrane.JPG

Name
Email Id
Mobile

Exam
Target Year

Related Resources
Molal Elevation Constant

Molal elevation constant : Molal elevation...

Determination of Solubility

Determination of solubility : The solubility of...

Molality

Molality: It is defined as the number of the moles...

Solvent and Solute

Solvent and Solute When two or more chemically...

Partial Pressure of Immiscible Liquids

Partial pressure of immiscible liquids Let...

Solved Examples of Molecular Mass

Solved Examples of Molecular Mass Example:...

Solutions of Solids in Liquids

SOLUTIONS OF SOLIDS IN LIQUIDS Solutions of this...

Solutions of Liquids in Liquids

SOLUTIONS OF LIQUIDS IN LIQUIDS When one liquid...

Relation-Osmotic Pressure and Vapour Pressure

Relation between osmotic pressure and vapour...

Molarity

Molarity (Molar concentration): It is defined as...

Lowering in Vapour Pressure

LOWERING IN THE VAPOUR PRESSURE When a...

Relationship-Molality and Molarity

Relationship between molality and molarity:...

Reverse Osmosis

REVERSE OSMOSIS When a solution is separated from...

Solutions of Gases in Liquids

SOLUTIONS OF GASES IN LIQUIDS All gases are...

Types of Solutions

TYPES OF SOLUTIONS All the three states of matter...

Methods of Expressing Concentration of Solution

Methods of Expressing the Concentration of a...

Vant Hoff Theory of Dilute Solutions

VAN’T HOFF THEORY OF DILUTE SOLUTIONS...

Determination of Molecular Masses

DETERMINATION OF MOLECULAR MASSES In the case of...

Determination of Osmotic Pressure

Determination of osmotic pressure: Berkeley and...

Raoults Law

Raoult’s Law Vapour Pressure Consider a pure...

Ideal Solutions

Ideal solutions : An ideal solution one in which...

Comparison-Ideal and Non-Ideal Solutions

Comparison Between Ideal and Non-ideal Solutions...

Example based on Lowering in Vapour Pressure

Example based on Lowering in the Vapour Pressure...

Normality

Normality: It is defines as the number of gram...

Ostwald and Walker Method

Ostwald and Walker Method Measurement of relative...

Depression of Freezing Point

DEPRESSION OF FREEZING POINT (CRYOSCOPY) Freezing...

Examples on Ideal and Non-ideal Solutions

Examples on Ideal & Non Ideal Solutions...

Theory of Fractional Distillation

THEORY OF FRACTIONAL DISTILLATION The process of...

Solved Examples

Solved Examples Question 1: Arginine vasopressin...

Solved Examples of Depression of Freezing Point

SOME SOLVED EXAMPLES Example: On dissolving 10.8...

Solubility

Solubility Solubility of a substance is its...

Mole Fraction

Mole fraction: This method is used when the...

Elevation of Boiling Point

ELEVATION OF BOILING POINT (EBULLIOSCOPY) : The...