Chemistry of Solutions and Colligative Properties
1. Understanding the Van’t Hoff Factor
The Van’t Hoff factor (i) is defined as the ratio of the actual number of particles present in a solution after dissociation or association to the number of particles originally dissolved.
It helps explain the abnormal values of colligative properties caused by dissociation or association of solute molecules.
Expression:
Example: When NaCl dissolves in water, it dissociates into Na⁺ and Cl⁻ ions, so the number of particles increases and i ≈ 2.
2. Isotonic Solutions and Osmotic Pressure
Two solutions having the same osmotic pressure at the same temperature are called isotonic solutions.
When two isotonic solutions are separated by a semipermeable membrane, no net flow of solvent occurs across the membrane. This happens because the osmotic pressure on both sides is equal, so the solvent movement balances.
Example: A 0.9% sodium chloride solution is isotonic with human blood. Therefore, red blood cells neither shrink nor swell in this solution.
3. Principles of Reverse Osmosis
Reverse osmosis is the process in which pressure greater than the osmotic pressure is applied to a solution to force the solvent to move from a concentrated solution to a dilute solution through a semipermeable membrane.
This process is opposite to normal osmosis, where solvent naturally moves from a dilute solution to a concentrated solution. Reverse osmosis is widely used for the purification of water, especially in RO water purifiers to remove salts and impurities.
4. Oxygen Solubility in Cold vs. Warm Water
Cold water is more suitable for aquatic organisms because it contains more dissolved oxygen than warm water.
The solubility of gases in liquids decreases with an increase in temperature. Therefore, warm water holds less oxygen, which is required by aquatic organisms for respiration. As a result, aquatic animals such as fish can survive better in cold water where the oxygen concentration is higher.
5. Relative Lowering of Vapour Pressure
Relative lowering of vapour pressure is defined as the ratio of the lowering of vapour pressure of a solution to the vapour pressure of the pure solvent.
When a non-volatile solute is dissolved in a solvent, the vapour pressure of the solvent decreases.
Mathematical expression:
Where:
P⁰ = vapour pressure of pure solvent
P = vapour pressure of solution
Relative lowering of vapour pressure is a colligative property because it depends only on the number of solute particles.
6. Why Azeotropic Mixtures Cannot Be Separated
Azeotropes are mixtures of liquids that boil at a constant temperature and have the same composition in both liquid and vapour phases.
Because the vapour formed during boiling has the same composition as the liquid, distillation cannot change the composition of the mixture. Therefore, the components of an azeotropic mixture cannot be separated by simple distillation.
Example: Ethanol and water mixture.
7. Characteristics of Ideal Solutions
An ideal solution is one that obeys Raoult’s law over the entire range of concentration.
Characteristics:
- ΔHmix = 0 (no heat is absorbed or evolved during mixing)
- ΔVmix = 0 (no change in volume on mixing)
- Intermolecular forces between unlike molecules are similar to those between like molecules.
Example: Benzene and toluene.
8. Defining Colligative Properties
Colligative properties are those properties of dilute solutions that depend only on the number of solute particles present in the solution and not on the nature of the solute.
These properties arise because the presence of solute particles affects the physical behavior of the solvent. The four important colligative properties are:
- Relative lowering of vapour pressure
- Elevation of boiling point
- Depression of freezing point
- Osmotic pressure
These properties are useful in determining the molar mass of solutes.
9. Raoult’s Law for Ideal Solutions
Raoult’s law states that the partial vapour pressure of each component in an ideal solution is directly proportional to its mole fraction in the solution.
Mathematically:
Where:
P_A = partial vapour pressure of component A
x_A = mole fraction of A in solution
P°_A = vapour pressure of pure component A
The total vapour pressure of the solution is equal to the sum of the partial vapour pressures of all components.
10. Positive Deviation from Raoult’s Law
A solution shows positive deviation from Raoult’s law when the vapour pressure of the solution is greater than that predicted by Raoult’s law.
This occurs because the attractive forces between different molecules are weaker than those between similar molecules. As a result, molecules escape easily into the vapour phase, increasing vapour pressure.
Example: Ethanol and acetone mixture.
11. Negative Deviation from Raoult’s Law
A solution shows negative deviation from Raoult’s law when the vapour pressure of the solution is lower than predicted by Raoult’s law.
This occurs because the attractive forces between unlike molecules are stronger than those between like molecules. Therefore, fewer molecules escape into the vapour phase, decreasing vapour pressure.
Example: Acetone and chloroform mixture.
12. Practical Applications of Henry’s Law
Henry’s law states that the solubility of a gas in a liquid is directly proportional to the pressure of the gas above the liquid.
Applications:
- Manufacturing of soft drinks: CO₂ is dissolved in beverages under high pressure.
- Scuba diving: Divers use special gas mixtures to avoid nitrogen bubbles in the blood.
- High altitude breathing: Low pressure reduces oxygen solubility in the blood.
13. Differences Between Molarity and Molality
Molarity (M): Molarity is defined as the number of moles of solute dissolved in one litre of solution.
Molality (m): Molality is defined as the number of moles of solute present in one kilogram of solvent.
Note: Molality is independent of temperature, whereas molarity changes with temperature.
14. Definition of Boiling Point
Boiling point is the temperature at which the vapour pressure of a liquid becomes equal to the external atmospheric pressure. At this temperature, the liquid changes into vapour throughout the liquid.
15. Definition of Freezing Point
Freezing point is the temperature at which a liquid changes into a solid and both phases remain in equilibrium. The addition of solute lowers the freezing point of a solvent.
16. Understanding Vapour Pressure
Vapour pressure is the pressure exerted by vapour in equilibrium with its liquid in a closed container at a given temperature. It depends mainly on temperature and the nature of the liquid.
17. Kohlrausch’s Law of Ion Migration
Kohlrausch’s law states that at infinite dilution, each ion contributes independently to the total molar conductivity of the electrolyte. Thus, the molar conductivity of an electrolyte at infinite dilution is the sum of the molar conductivities of its ions.