Colligative Properties of Solutions

Colligative Properties of Solution

• Colligative properties depend mainly on the number of particles in a solution.
• These properties are co-related, and any change in one of them will be accompanied by a corresponding change in the others.

Osmotic Pressure

• Osmosis: the movement of a solvent through a semi-permeable membrane from a solution of lower concentration to a solution of higher concentration.
• Osmotic pressure is the pressure that must be applied to the solution to prevent the passage of the solvent through a perfect semi-permeable membrane.
• Osmotic pressure can be expressed by Van't Hoff equation: PV = nRT.

Example of Osmotic Pressure Calculation

• To calculate the osmotic pressure of 50g of dextrose in 1000ml of water at 25°C: P = (0.28)(0.082)(298) / 1 = 6.84 atm.

Modifications for Electrolytes

• The Van't Hoff equation must be modified to allow for the effect of ionic dissociation, which increases the number of particles in the solution.

Effects of Osmotic Pressure on Red Blood Cells

• Hypotonic solution: lower salt concentration and lower osmotic pressure than the red blood cell contents, causing water to enter the cells and leading to swelling and hemolysis.
• Isotonic solution: equal salt concentration and osmotic pressure to the red blood cell contents, maintaining cell shape and size.

Other Colligative Properties

• Vapor pressure lowering: caused by the reduction of surface area available for escaping solvent molecules due to the presence of solute particles.
• Boiling point elevation: a solution requires a higher temperature to reach its boiling point than the pure solvent due to vapor pressure lowering.
• Freezing point depression: a solution has a lower freezing point than the pure solvent due to the disruption of solvent-solvent interactions by solute particles.

Applications of Colligative Properties

• Method of adjusting tonicity: adding sodium chloride or a substance to lower the freezing point of the solution to -0.52°C, making it isotonic with body fluids.
• Cryoscopic method: adjusting the freezing point of a solution to -0.52°C by adding sodium chloride, making it isotonic with blood.
• Sodium Chloride Equivalent method: using the dissociation factor (i) to adjust the concentration of a solution to make it isotonic with body fluids.