Chemistry Solutions: Vapor Pressure and Colligative Properties

Class Structure for Problem Design

• Students must design one test problem.
• Format: Can be multiple choice or free response.
• Include a detailed solution step-by-step.
• Load both the problem and the solution in Canvas before class.
• Attendance requirement: Must attend class to receive bonus points; submitting online without attendance does not count.

Peer Review Activities

• First half of the class is designated for peer-to-peer review.
• Engage in discussions with peers about their problems.
• Move around and review at least 10 questions to enhance understanding.
• The second half will include discussion on three comprehensive questions covering thermochemistry, ideal gas law, and stoichiometry.
• Expectation of student involvement in problem-solving for extra credit.

Understanding Molarity vs. Molality

• Molarity: $M = rac{ ext{moles of solute}}{ ext{liters of solution}}$.
• Dependent on volume, which can change with temperature and pressure.
• Molality: $m = rac{ ext{moles of solute}}{ ext{kg of solvent}}$.
• Independent of temperature and pressure; less affected by volume changes.
• Used in colligative properties due to stability under different conditions.

Colligative Properties and Vapor Pressure

• Vapor Pressure: Refers to pressure at which a liquid's vapor is in equilibrium with its liquid form.
• Higher vapor pressure indicates weaker intermolecular forces, leading to lower boiling points.
• Comparison between solutions:
• Pure solvent vs. solvent with nonvolatile solute.
• Presence of solute decreases vapor pressure of solvent.

Raoult's Law

• Formula: $P{ ext{solution}} = ext{X}{ ext{solvent}} imes P^{ ext{0}}_{ ext{solvent}}$
• $P_{ ext{solution}}$: vapor pressure of the solution.
• $ ext{X}_{ ext{solvent}}$: mole fraction of the solvent.
• $P^{ ext{0}}_{ ext{solvent}}$: vapor pressure of the pure solvent.
• The mole fraction is calculated based on the total moles of solute and solvent.
• Understand differences between Raoult’s law and Henry’s law.
• Henry’s law applies to gases in liquids, specifically focusing on solubility.

Colligative Property Examples

• Boiling point elevation and freezing point depression.
• Boiling point/elevation: $ΔTb = Kb imes m$
  • $K_b$: boiling point elevation constant.
• Freezing point depression: $ΔTf = Kf imes m$
  • $K_f$: freezing point depression constant.
• Application: Addition of salt lowers the freezing point of water (common for roads to prevent icing).

Ideal vs. Non-Ideal Solutions

• Ideal Solutions: Components behave according to Raoult's law.
• Mixtures with similar intermolecular forces show ideal behavior.
• Non-Ideal Solutions: Deviations occur due to differing interactions between solute and solvent, resulting in either positive or negative deviations.
• Positive Deviation: Vapor pressure higher than expected; weak solute-solvent interaction (e.g., ethanol and hexane).
• Negative Deviation: Vapor pressure lower than predicted; strong interactions such as dipole-dipole interactions (e.g., acetone and water).

Reverse Osmosis

• Definition: Movement of solvent from low to high solute concentration through a semi-permeable membrane.
• Essential for desalination processes and treatment of contaminated water.
• Requires high pressure to operate, often around 50 atmospheres for effective desalination.

Application in Real-World Contexts

• Importance of understanding colligative properties for various applications, including understanding solutions in biological and engineering contexts.
• Discussion on practical importance, especially concerning water purification and the necessity for clean water supply in response to population growth.