This exam focus covers Exam Focus within Gas Volume for GCSE Chemistry. Topic 19: Gas Volume It is section 9 of 11 in this topic. Treat this as a marking guide for what examiners are looking for, not just a fact list.
Topic position
Section 9 of 11
Practice
20 questions
Recall
20 flashcards
🎯 Exam Focus
Frequently Examined
Gas volume calculations appear on most GCSE Chemistry papers. Common question formats include:
- Direct conversion: "Calculate the volume of X moles of gas at RTP" (1-2 marks)
- Equation-based: Given a balanced equation and mass of one reactant, find the volume of gas produced (3-4 marks)
- Multi-step: Mass → moles → mole ratio → gas volume, sometimes with unit conversion (4-5 marks)
- Reverse: Given a volume of gas, find the mass of another reactant (3-4 marks)
Key phrase in questions: "at room temperature and pressure" or "at RTP" signals you must use 24 dm³/mol. If the question gives a different temperature, it will provide a different molar volume value — use that instead.
Quick Check: What volume of gas is produced by 0.25 moles of CO₂ at RTP?
Volume = moles × 24 = 0.25 × 24 = 6 dm³. (Or 6000 cm³ if the question asks for cm³.)
Quick Check: A student collects 4800 cm³ of hydrogen gas at RTP. How many moles of hydrogen is this?
First convert: 4800 cm³ ÷ 1000 = 4.8 dm³. Then moles = volume ÷ 24 = 4.8 ÷ 24 = 0.2 mol. Always convert cm³ to dm³ before dividing by 24.
Quick Check: What mass of calcium carbonate is needed to produce 2.4 dm³ of CO₂ at RTP? (CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂, Mr of CaCO₃ = 100)
Moles of CO₂ = 2.4 ÷ 24 = 0.1 mol. From equation: 1 mol CaCO₃ produces 1 mol CO₂, so 0.1 mol CaCO₃ needed. Mass = 0.1 × 100 = 10 g.