Gases
Gases
This domain tests the gas laws, the ideal gas equation, kinetic theory and partial pressures. Questions often combine a gas calculation with stoichiometry or ask you to explain deviations from ideal behaviour.
Core rules
- Boyle: P1V1 = P2V2 at constant T. Charles: V1/T1 = V2/T2 at constant P. Gay-Lussac: P1/T1 = P2/T2 at constant V.
- Combined gas law: P1V1/T1 = P2V2/T2. Temperature must be in kelvin.
- Ideal gas equation: PV = nRT, with R = 8.314 J mol^-1 K^-1 when P is in Pa and V in m^3.
- At STP (273 K, 1 × 10^5 Pa), molar volume is 22.4 L.
- Dalton's law: total pressure is the sum of partial pressures. Kinetic theory assumes random motion, negligible particle volume and no intermolecular forces.
Worked example
Calculate the volume of 0.500 mol of an ideal gas at 27 °C and 100 kPa. T = 27 + 273 = 300 K. P = 100 000 Pa. V = nRT/P = (0.500 × 8.314 × 300)/100 000 = 0.0125 m^3 = 12.5 L.
Traps
Using degrees Celsius instead of kelvin is the most common error. Convert kPa to Pa and mL to m^3 before using R = 8.314. Real gases deviate most at high pressure and low temperature, where particle volume and attractions matter. Gas volumes are not conserved in reactions; use mole ratios.