Knowledge Organiser: Gas Volumes
Key Terms
- RTP: 20°C and 1 atmosphere pressure
- Molar gas volume: 24 dm³/mol at RTP
- Volume = n × 24 (dm³)
- Moles = V ÷ 24
- 1 dm³ = 1000 cm³
Must-Know Facts
- All gases: 24 dm³/mol at RTP
- Particle size does not affect molar volume
- Multi-step route: mass → moles → ratio → volume
- Use 24 (not 22.4) for GCSE calculations
- Always convert cm³ ÷ 1000 to get dm³ first
Key Equations
- Volume (dm³) = moles × 24
- Moles = volume (dm³) ÷ 24
- 1 dm³ = 1000 cm³ (always convert before calculating)
- Molar gas volume at RTP = 24 dm³/mol
Common Mistakes
- Using 22.4 dm³/mol instead of 24: At GCSE, RTP is 20°C and 1 atm — molar volume is 24 dm³/mol (22.4 is used at 0°C/STP, not GCSE)
- Forgetting to convert cm³ to dm³: Divide cm³ by 1000 before using volume = moles × 24
- Thinking heavier gases occupy less volume: All gases occupy the same molar volume at RTP — 24 dm³ per mole regardless of Mr
- Skipping the mole ratio step: In multi-step calculations, use the equation ratio to convert moles of one gas to moles of another before calculating volume
Memory Aid
- "24 is the magic number at RTP" — one mole of any gas = 24 dm³. Think of it as "two dozen" — 24 dm³, just like 24 hours in a day or 24 items in two dozen.
- The gas triangle: Draw a triangle with Volume (V) at the top, Moles (n) bottom-left, and 24 bottom-right. Cover V → n × 24. Cover n → V ÷ 24. This is the gas equivalent of the moles triangle.
- Unit check mantra: "cm³? Divide by 1000 to get dm³ FIRST." Always convert before using V = n × 24.
- Multi-step route: "mass → moles → ratio → moles of gas → volume" — always follow this sequence for complex questions.