Boiling Point Elevation

Introduction to Boiling Point Elevation

Boiling point elevation is a colligative property observed when a solute is added to a pure solvent, resulting in an increase in the boiling point of the solvent. This phenomenon is important in understanding how solutions behave compared to pure substances.

• Normal Boiling Point: The boiling point of the pure solvent before the addition of any solute.

• Boiling Point of Solution: The boiling point of the solvent after the addition of the solute.

Key Concepts and Definitions

• Colligative Properties: Properties that depend on the number of solute particles in a solution, not their identity. Boiling point elevation is one such property.

• Boiling Point Elevation (ΔTb): The increase in boiling point observed when a non-volatile solute is dissolved in a solvent.

• Molality (m): The number of moles of solute per kilogram of solvent.

• van't Hoff Factor (i): The number of particles into which a solute dissociates in solution. For non-electrolytes, i = 1; for electrolytes, i is greater than 1.

Boiling Point Elevation Equation

The change in boiling point can be calculated using the following equation:

• = Boiling point elevation (°C)

• = van't Hoff factor (number of particles the solute splits into)

• = Molal boiling point elevation constant (°C·kg/mol), specific to each solvent

• = Molality of the solution (mol/kg)

Constants for Common Solvents

van't Hoff Factor (i)

• If a compound is covalent, nonvolatile, or non-ionic, then its van't Hoff factor is equal to 1.

• For ionic compounds, i equals the total number of ions produced per formula unit in solution.

Example Calculation

Example: Calculate the boiling point of a 3.71 m aqueous CaBr2 solution.

• CaBr2 dissociates into 3 ions (1 Ca2+ and 2 Br-), so i = 3.

• For water, °C·kg/mol.

• Molality, m = 3.71 mol/kg.

°C

• New boiling point = 100.0 °C + 5.70 °C = 105.7 °C

Practice Problems

• Practice 1: An ethylene glycol solution contains 25.2 g of ethylene glycol (C2H6O2) in 99.5 mL of water. Determine the change in boiling point. Assume a density of 1.00 g/mL for water.

• Practice 2: Pure water boils at 100°C. What is the new boiling point of water after the addition of 13.12 g aluminum chloride, AlCl3, to 615 g water?

• Practice 3: What is the molality of glucose in an aqueous solution if the boiling point of the solution is 103.15°C?

• Practice 4: Carbon dioxide is dissolved in 722 mL of benzene with a density of 1.59 g/mL. What mass of carbon dioxide would you add to make the boiling point of the solution 104.7°C?

Summary Table: Steps for Boiling Point Elevation Calculations

Additional info: The practice problems provided are typical for GOB Chemistry and reinforce the application of the boiling point elevation formula in various contexts, including both ionic and molecular solutes.