Boiling Point Elevation: Concepts and Calculations

Boiling Point Elevation

Concept Overview

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 differently from pure substances.

• Normal Boiling Point: The temperature at which a pure solvent boils under standard atmospheric pressure.

• Boiling Point of Solution: The temperature at which a solution boils, which is higher than that of the pure solvent due to the presence of a solute.

Key Equation: Boiling Point Elevation

The increase in boiling point, ΔTb, is given by:

$\Delta T_b = i K_b m$

• ΔTb: Boiling point elevation (°C)

• i: van 't Hoff factor (number of particles the solute splits into in solution)

• Kb: Molal boiling point elevation constant (°C·kg/mol) for the solvent

• m: Molality of the solution (mol solute/kg solvent)

Note: For non-electrolytes (do not dissociate), i = 1. For electrolytes, i equals the number of ions produced per formula unit.

Example Calculation

Example: Calculate the boiling point of a 2.71 m aqueous CaCl2 solution.

• CaCl2 dissociates into 3 ions (i = 3).

• Use the equation $\Delta T_b = i K_b m$ with the appropriate Kb for water.

Application: This calculation is essential in laboratory and industrial settings where precise boiling points are required for solution preparation.

Practice Problems

• Problem 1: An ethylene glycol solution contains 25.2 g of ethylene glycol (C2H6O2) in 101.5 mL of water. Determine the change in boiling point. Assume a density of 1.00 g/mL for water.

• Problem 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?

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

• Problem 4: Carbon dioxide is dissolved in 722 mL of tetrachloroethene with a density of 1.58 g/mL. What mass of carbon dioxide would you add to raise the boiling point of the solution to 104.1°C?

Summary Table: Boiling Point Elevation Variables

Variable	Definition	Unit
ΔTb	Boiling point elevation	°C
i	van 't Hoff factor	unitless
Kb	Boiling point elevation constant	°C·kg/mol
m	Molality	mol/kg

Additional info: Boiling point elevation is one of several colligative properties, which also include freezing point depression, vapor pressure lowering, and osmotic pressure. These properties depend only on the number of solute particles in solution, not their identity.