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OSMOTIC PRESSURE

Course Title: Physical Chemistry IICourse Code: ACCE 260Applied Chemistry & Chemical Engineering

SHAIKH ASHRAFUL ALAM Dept. of Applied Chemistry & Chemical Engineering BSMRSTU,Gopalgang,Bangladesh

Osmosis

The passage of solvent into solution or from more dilute to concentrated solution when the two are separated from each other by a semipermeable membrane is known as osmosis.

Diffusion of water through the semi permeable membrane from a solution of lower concentration towards a solution of higher concentration

Pure Solvent

Concentrated Solution

semi permeable membrane

Solvent molecule

Solute molecule

Osmotic PressureOsmotic pressure is the minimum pressure which needs to be applied

to a solution to prevent interior flow of water across a semipermeable membrane.

orThe pressure required to stop osmosis.

Osmosis and Osmotic Pressure

Determination of Osmotic Pressure

Pfeffer’s MethodBerkely and Harthey’s MethodA Modern Osmometer.

Important term of osmosis & Osmotic pressureIsotonic: Solutions have equal concentration of solute, and so equal osmotic

pressure.

Hypertonic: Solution with higher concentration of solute.

Hypotonic: Solution with lower concentration of solute.

Isotonic Hypertonic Hypotonic

Law of osmotic pressure

Boyle-van’t Hoff law for solutions

Charles’- Van’t Hoff law for solution

Combination of two law we get Van’t Hoff law for solution ( here is osmotic pressure)

Calculating osmotic pressure

The ideal gas law states

But n/V = M and so

Where is the molar concentration of particles and is the osmotic pressure, isVan’t Hoff factor

𝑃𝑉=𝑛𝑅𝑇𝜋=𝑖𝑀𝑅𝑇

Relation between osmatic pressure and other colligative properties

Here, is the elevation in the boiling or depression in the freezing point. is the boiling or freezing point of the solvent. is the enthalpy change during phase change. is the mole fraction of solute in solution.

Osmotic Pressure – Applications:• Osmotic pressure effects normally move solvent molecules from a dilute to a concentrated

solution. In water desalination a large “external” pressure is used to push solvent molecules (water) from sea water through a membrane to produce drinking water. The process is shown below.

Desalination of saltwater by reverse osmosis

Importance of Osmosis and Osmotic Pressure

Oncotic pressure of blood plasma

Formation of tissue fluid

Regulation of cell volume