CHEM Lecture 6

Overview

This lecture covers the main gas laws—Avogadro's, Boyle's, and Charles's Laws—partial pressure concepts, the ideal gas equation, and the differences between ideal and real gases, with example problem-solving strategies.

Key Gas Laws

• Avogadro's Law: Volume (V) is directly proportional to the number of moles (n) at constant T and P; V/n = constant.
• Boyle's Law: Pressure (P) is inversely proportional to volume (V) at constant T and n; PV = constant, so P₁V₁ = P₂V₂ for initial/final states.
• Charles's Law: Volume (V) is directly proportional to temperature (T in Kelvin) at constant P and n; V/T = constant, so V₁/T₁ = V₂/T₂ for changes.

Partial Pressure & Gas Mixtures

• Dalton's Law: Total pressure equals the sum of partial pressures of individual gases in a mixture.
• Gases in a mixture act independently and do not affect each other's pressure if they don’t react.
• Partial pressure can be calculated by (mole of gas A / total moles) × total pressure.
• Gases with the same number of moles have the same partial pressure under identical conditions.

Ideal Gas Equation & Derived Forms

• Ideal Gas Law: PV = nRT, where R is the gas constant (8.314 kPa·dm³/mol·K or 0.0821 atm·L/mol·K).
• Ensure units for P, V, T, and n match R’s units.
• Number of moles: n = mass / molar mass.
• Derived form: (mass / molar mass) = PV / RT; density can be integrated into the equation.

Ideal Gas vs. Real Gas

• Gases behave ideally at low pressure (high volume) and high temperature (weak intermolecular forces, negligible particle size).
• Real gases deviate at high pressure (small volume) and low temperature (significant particle size, stronger forces).
• Kinetic Molecular Theory assumptions only apply to ideal gases.

Sample Problem Approaches

• Use PV = nRT when all variables are given (no changes).
• Use gas laws (Boyle’s, Charles’s) when comparing before/after conditions.
• Always convert temperature to Kelvin: K = °C + 273.
• Make sure units are consistent (e.g., kPa with dm³, atm with L).

Key Terms & Definitions

• Partial Pressure — Pressure exerted by a single gas in a mixture.
• Ideal Gas — Hypothetical gas following all assumptions of kinetic molecular theory.
• Real Gas — Actual gases, deviating from ideal behavior under certain conditions.

Action Items / Next Steps

• Complete and review questions 5–10 on gas laws as practice.
• Prepare and submit lab/practical reports on time.
• Next quiz: 19th September (covers Chapters 1 and 2).
• Bring and familiarize yourself with a scientific calculator.