Che 252

Colligative Properties

• Definition: Colligative properties are properties that depend on the number of solute particles in a solution and not on their identity.

• Distinction: Unlike non-colligative properties (such as color, density, and order), the specific identity of the substance is not important as long as the nature (ionic or covalent) is considered.

Types of Colligative Properties

• Key Colligative Properties:

  • Vapor pressure elevation

  • Boiling point elevation

  • Freezing point depression

  • Osmotic pressure

Electrolytes vs Non-Electrolytes

• Electrolytes:

  • Substances that dissociate into ions in solution.

  • Example: Sodium chloride (NaCl) and potassium chloride (KCl) both are ionic and produce two ions, therefore behave similarly in terms of colligative properties.

• Non-Electrolytes:

  • Do not dissociate into ions, resulting in one particle per molecule.

  • Examples: Fructose and glucose; both do not dissociate and thus have the same colligative properties despite being different compounds.

Key Formulas and Concepts

• Vapor Pressure of Solutions:

  • Determined using Raoult's Law by knowing:

    • Vapor pressure of the pure solvent.

    • Mole fraction of the solvent (Mole fraction of solute is not used in this calculation).

• Common Mistakes:

  • Students often mistakenly calculate mole fraction of the solute instead of the solvent, leading to incorrect results.

  • Important to consider multiple components if two or more liquids are present, calculating each component’s vapor pressure.

Example Problem: Vapor Pressure Calculation

• Given:

  • Vapor pressure of 2-methylhexane at 15 °C = 37.986 torr.

  • Mixture contains 78 g of 2-methylhexane and 15 g of naphthalene (assumed non-volatile).

• Steps to Solve:

  1. Identify solvent (2-methylhexane) and calculate its mole fraction.

  2. Calculate moles:

     • Moles of 2-methylhexane = ( \frac{78 g}{100.2 g/mol} )

     • Moles of naphthalene = ( \frac{15 g}{128.17 g/mol} )

  3. Use the total moles to find mole fraction of the solvent:

     • Mole fraction (2-methylhexane) = ( \frac{moles\ of\ solvent}{total\ moles} )

  4. Apply Raoult's Law:

     • Pressure of solution = Mole fraction of 2-methylhexane × Vapor pressure of pure 2-methylhexane.

• Solution Result:

  • Final pressure of the solution calculated to be approximately 33 torr.