How to Calculate Boiling Point Elevation: Worked Examples

🧪 Quick Study Guide

• 🌡️ Boiling point elevation is a colligative property, meaning it depends on the number of solute particles in a solution, not their identity.
• ⚗️ The formula for boiling point elevation is: $$\Delta T_b = i \cdot K_b \cdot m$$, where:
• $\Delta T_b$ is the boiling point elevation.
• $i$ is the van't Hoff factor (number of particles the solute dissociates into).
• $K_b$ is the ebullioscopic constant (boiling point elevation constant) of the solvent.
• $m$ is the molality of the solution (moles of solute per kilogram of solvent).
• 💧 Molality (m) is calculated as: $$m = \frac{\text{moles of solute}}{\text{kilograms of solvent}}$$.
• 🔑 Remember to always use the correct units: $K_b$ is in °C kg/mol, and molality is in mol/kg.
• 💡 The new boiling point of the solution is: $$T_{b(solution)} = T_{b(pure \ solvent)} + \Delta T_b$$.

Practice Quiz

1. Question 1: What is a colligative property?
   1. A) A property that depends on the type of solute.
   2. B) A property that depends on the amount of solvent.
   3. C) A property that depends on the number of solute particles.
   4. D) A property that depends on the identity of the solvent.
2. Question 2: What does the van't Hoff factor (i) represent?
   1. A) The molality of the solution.
   2. B) The boiling point elevation constant.
   3. C) The number of particles a solute dissociates into.
   4. D) The change in temperature.
3. Question 3: What is the formula for calculating molality (m)?
   1. A) $$m = \frac{\text{kilograms of solvent}}{\text{moles of solute}}$$
   2. B) $$m = \frac{\text{moles of solute}}{\text{liters of solution}}$$
   3. C) $$m = \frac{\text{moles of solute}}{\text{kilograms of solvent}}$$
   4. D) $$m = \frac{\text{grams of solute}}{\text{kilograms of solvent}}$$
4. Question 4: If $K_b$ for water is 0.512 °C kg/mol, and you have a 1 molal solution of NaCl in water (i=2), what is the boiling point elevation?
   1. A) 0.256 °C
   2. B) 0.512 °C
   3. C) 1.024 °C
   4. D) 2.048 °C
5. Question 5: What are the correct units for the ebullioscopic constant ($K_b$)?
   1. A) mol/kg
   2. B) °C/mol
   3. C) °C kg/mol
   4. D) kg/°C
6. Question 6: You dissolve 58.44 grams of NaCl (i=2) in 1 kg of water. What is the molality of the solution? (Molar mass of NaCl = 58.44 g/mol)
   1. A) 0.5 m
   2. B) 1 m
   3. C) 2 m
   4. D) 0.1 m
7. Question 7: If pure water boils at 100 °C, and a solution has a boiling point elevation of 1.5 °C, what is the boiling point of the solution?
   1. A) 98.5 °C
   2. B) 100 °C
   3. C) 101.5 °C
   4. D) 102 °C