Tutorial 5 — Solutions and Raoult's Law

Learning Outcomes

Understand Raoult's Law and ideal solutions
Calculate vapor pressure of solutions
Identify deviations from Raoult's Law
Calculate colligative properties

Part A: Raoult's Law

Question 1

a) State Raoult's Law. What assumptions are made about the solution?

b) A solution contains 40.0 g of glucose ($C_6H_{12}O_6$) dissolved in 500 g of water at 25°C. Calculate:

The vapor pressure of the solution
The vapor pressure lowering

Given: Vapor pressure of pure water at 25°C = 23.8 mmHg

Question 2

Benzene ($C_6H_6$) and toluene ($C_7H_8$) form an ideal solution. At 30°C, the vapor pressures of pure benzene and toluene are 120 mmHg and 37 mmHg respectively.

a) Calculate the partial vapor pressures of benzene and toluene in a solution containing 2.0 mol of benzene and 3.0 mol of toluene.

b) Calculate the total vapor pressure of the solution.

c) Calculate the mole fraction of benzene in the vapor phase.

Part B: Non-Ideal Solutions

Question 3

A solution of carbon disulfide ($CS_2$) and acetone ($CH_3COCH_3$) shows positive deviation from Raoult's Law.

a) Explain what positive deviation means in terms of intermolecular forces.

b) The experimental total vapor pressure of the solution is 645 torr, which is higher than the calculated total vapor pressure of 434.2 torr. This suggests that the solution is not ideal.

Extracted explanation: For an ideal solution, the total vapour pressure is determined purely by Raoult's Law. The fact that the experimental vapour pressure is higher than expected indicates that the intermolecular interactions between the components are weaker than those between the same type of molecules (i.e., CS-CS or CH₃COCH₃-CH₃COCH₃).

c) Sketch a vapor pressure-composition diagram for this system.

Question 4

a) Identify the characteristics of a solution showing negative deviation from Raoult's Law:

Extracted answer:

Stronger interactions between the components in the solution than within pure substances

Lower vapor pressure compared to ideal predictions

Exothermic heat of mixing as the solution forms

Solubility decreases with temperature

b) Give an example of a solution that shows negative deviation and explain why.

Part C: Colligative Properties

Question 5

a) Define colligative properties. List the four main colligative properties.

b) Calculate the boiling point elevation when 10.0 g of NaCl is dissolved in 500 g of water.

Given: $K_b$ for water = 0.512 °C·kg/mol

c) Why is the observed boiling point elevation different from the calculated value for electrolyte solutions?

Question 6

a) A solution containing 2.50 g of a non-volatile solute in 100 g of benzene freezes at 4.92°C. Calculate the molar mass of the solute.

Given: Freezing point of pure benzene = 5.50°C, $K_f$ for benzene = 5.12 °C·kg/mol

b) Calculate the osmotic pressure of a 0.010 M CaCl₂ solution at 25°C.

Part D: Azeotropes

Question 7

a) What is an azeotrope? Explain the difference between minimum-boiling and maximum-boiling azeotropes.

b) Why can't azeotropic mixtures be separated by simple distillation?

c) Give an example of a minimum-boiling azeotrope.

Key Concepts

Raoult's Law — $P_{solution} = X_{solvent} \times P°_{solvent}$
Ideal Solution — Solution obeying Raoult's Law
Positive Deviation — Higher vapor pressure than predicted
Negative Deviation — Lower vapor pressure than predicted
Colligative Properties — Properties depending on number of solute particles
Azeotrope — Mixture with same composition in liquid and vapor phases

FAD1018 Tutorial 5 — Solutions and Raoult's Law