Gas Volumes — GCSE Chemistry Revision
Revise Gas Volumes for GCSE Chemistry. Step-by-step explanation, worked examples, common mistakes and exam-style practice aligned to AQA, Edexcel, OCR, WJEC, Eduqas, CCEA, Cambridge International (CIE), SQA, IB, AP.
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Go to Relative Formula MassWhat is Gas Volumes?
At the same temperature and pressure, equal volumes of different gases contain the same number of molecules. At room temperature and pressure (RTP), one mole of any gas occupies a volume of 24 dm³ (or 24,000 cm³). This relationship can be used to calculate gas volumes in reactions.
Board notes: Calculations involving gas volumes are a higher-tier topic for all boards. You need to be able to use the molar volume of a gas in calculations involving moles and balanced chemical equations.
Step-by-step explanationWorked example
What volume of carbon dioxide gas is produced at RTP when 2.5g of calcium carbonate (CaCO₃, Mr=100) decomposes? CaCO₃ → CaO + CO₂. 1. Moles of CaCO₃ = 2.5g / 100 = 0.025 mol. 2. Ratio of CaCO₃ to CO₂ is 1:1, so 0.025 mol of CO₂ is produced. 3. Volume of CO₂ = 0.025 mol x 24 dm³/mol = 0.6 dm³.
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Common mistakes
- 1Forgetting the molar gas volume value (24 dm³/mol at RTP). This is usually given in the exam, but it's good to know.
- 2Using mass instead of moles in the calculation. The formula is Volume = Moles x Molar Volume.
- 3Not ensuring the units are consistent. If you use dm³ for the molar volume, the calculated volume will also be in dm³.
Gas Volumes exam questions
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Step-by-step method
Step-by-step explanation
4 steps · Worked method for Gas Volumes
Core concept
At the same temperature and pressure, equal volumes of different gases contain the same number of molecules. At room temperature and pressure (RTP), one mole of any gas occupies a volume of 24 dm³ (or…
Frequently asked questions
What is RTP?
RTP stands for Room Temperature and Pressure. It is a standard set of conditions used for comparing gas volumes, typically defined as 20-25°C and 1 atmosphere of pressure.
Does the size of the gas molecules affect the volume?
No, at this level of chemistry, we assume that the volume of the gas molecules themselves is negligible compared to the volume the gas occupies. Therefore, one mole of any gas occupies the same volume under the same conditions.