Lecture Notes: Non-Volatile Solutes and Colligative Properties

Non-Volatile Solutes and Vapor Pressure

• Non-volatile solutes lower the vapor pressure of a solvent.
• Solution has lower vapor pressure than pure solvent.
• Phase Diagram Impact:
  • Pure solvent shows vapor pressure curve, freezing curve (solid-liquid transition), and sublimation curve.
  • Adding a non-volatile solute lowers vapor pressure, increasing boiling temperature required for solution.
  • Boiling point elevation and freezing point depression occur due to vapor pressure changes.

Boiling Point Elevation and Freezing Point Depression

• Equations:
  • Boiling Point Elevation: ( \Delta T_b = K_b \times m )
  • Freezing Point Depression: ( \Delta T_f = K_f \times m )
  • ( m ) is the molality of the solute.
• Van’t Hoff Factor (i):
  • Accounts for the number of particles an ionic compound produces.
  • Example: Sodium chloride (NaCl) has ( i \approx 2 ).
  • Adjusts for more particles than moles of substance.

Colligative Properties

• Depend on the number of particles, not their identity.
• Examples:
  • Boiling point elevation
  • Freezing point depression
  • Osmotic pressure
• Properties are primarily solvent-based.
• Raoult’s Law:
  • Mole fractions of the solvent are important.

Osmosis

• Definition: Movement of water through a semi-permeable membrane.
• Osmotic Pressure: Pressure caused by water moving into a solution.
  • Calculated using: ( \pi = M \times R \times T )
  • ( M ) is molarity, ( R ) is the ideal gas constant, ( T ) is temperature.
• Water Movement:
  • From lower to higher solute concentrations.

Isotonic, Hypotonic, and Hypertonic Solutions

• Isotonic: Equal flow of water in and out of the cell.
• Hypertonic: Water flows out of the cell, causing shrinkage.
• Hypotonic: Water flows into the cell, causing swelling.
• Osmolarity vs. Tonicity:
  • Osmolarity: Concentration of solute particles.
  • Tonicity: The effect of a solution on the volume of a cell.

Calculating Colligative Properties

• Example Problems:
  • Boiling Point Elevation:
    • For a 0.5 molal glucose solution, calculate elevation using glucose molecular properties and water’s boiling point constant.
  • Freezing Point Depression:
    • For a 1.74 molal naphthalene solution in carbon tetrachloride, calculate depression using freezing point constants.
  • Osmotic Pressure:
    • Calculate using hemoglobin concentration in solution.

Key Constants and Conversions

• Van’t Hoff Factor: Influences calculations for ionic compounds.
• Boiling/Freezing Point Constants (K_b/K_f): Essential for calculations.
• Ideal Gas Constant (R): Important for osmotic pressure and related calculations.

These notes summarize the key concepts and calculations related to non-volatile solutes, colligative properties, and their implications on phase diagrams and osmotic processes.