A semi-permeable membrane discriminates on the basis of molecular type Solvent molecules pass through Large molecules or ions are blocked
Oncotic or colloid osmotic pressure is a form of osmotic pressure exerted by proteins.
Course Code: ACCE 260
Applied Chemistry & Chemical
SHAIKH ASHRAFUL ALAM
Dept. of Applied Chemistry & Chemical Engineering
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
Osmotic pressure is the minimum pressure which needs to be applied
to a solution to prevent interior flow of water across a semipermeable
The pressure required to stop osmosis.
Determination of Osmotic Pressure
Berkely and Harthey’s Method
A Modern Osmometer.
Important term of osmosis & Osmotic
Isotonic: Solutions have equal concentration of solute, and so equal osmotic
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
π𝑉 = 𝑛𝑅𝑇 ( here 𝜋 is osmotic pressure)
Calculating osmotic pressure
The ideal gas law states
But n/V = M and so
Where 𝑀 is the molar concentration of particles
and 𝜋 is the osmotic pressure, 𝑖 isVan’t Hoff
𝑃𝑉 = 𝑛𝑅𝑇
𝜋 = 𝑖𝑀𝑅𝑇
Relation between osmatic pressure and
other colligative properties
𝑛1 + 𝑛2
∆𝑇𝑖 is the elevation in the boiling or depression in the freezing point.
𝑇𝑖 is the boiling or freezing point of the solvent.
∆𝐻𝑖 is the enthalpy change during phase change.
𝑥2 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
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