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gas calculations

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4 detailed 50-minute lessons with teaching scripts, worked examples, parent guides, and assessment criteria.

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Lesson Overview

Total Lessons: 4
Tier: Foundation and Higher
Duration: 50 minutes per lesson (200 minutes total)
Exam Boards: AQA, Edexcel, OCR, Eduqas, CCEA

Learning Objectives

Prerequisites

Materials & Equipment

Lesson 1: Introduction: gas calculations

Duration: 50 minutes

Starter Activity (5 minutes)

Quick Recall

Write down everything you already know about gas calculations. Then check against the key terms: Molar gas volume, Room temperature and pressure (RTP). Use a mini-whiteboard or paper.

Main Content (35 minutes)

Parent/Teacher Guide:
Before lesson: Read the script below. Pre-teach key vocab: Molar gas volume, Room temperature and pressure (RTP).
If stuck: Re-read the revision notes (link above), then break the content into smaller steps.
Extension: See the Stretch & Challenge ideas in Lesson 4.
Teaching Script (35 mins):
Mins 0-5 - Hook: "Today: gas calculations. By the end you will be able to answer exam questions on it unaided. It connects to the rest of Combined Science (Trilogy) because the ideas here recur across the spec."
Mins 5-20 - Direct Instruction: Work through the core ideas below one at a time; after each, ask your student to explain it back in their own words.
Mins 20-30 - Guided Practice: Model the worked example together, then let your student attempt the first practice question with guidance.
Mins 30-35 - Independent Practice: 2-3 practice questions from Lesson 3 below, with immediate feedback.
First Look

Start with the revision notes summary, then attempt: Higher What volume does 0.5 mol of chlorine gas occupy at RTP?

Plenary (5 minutes)

Check Out

Your student states one thing they learned and one question they still have about gas calculations.

Lesson 2: Core Concepts: gas calculations

Duration: 50 minutes

Starter Activity (5 minutes)

Review Previous Lesson

Quick recap: write 3 key points from Lesson 1 on gas calculations. Check them against the notes below.

Main Content (35 minutes)

Molar gas volume: The volume occupied by one mole of any gas at room temperature and pressure (RTP). At RTP (20°C and 1 atm), one mole of any gas occupies 24 dm³ (or 24 000 cm³).
Room temperature and pressure (RTP): 20°C and 1 atmosphere pressure. At RTP, equal amounts in moles of any gas occupy the same volume.

Practice (10 minutes)

Q: Higher What volume does 0.5 mol of chlorine gas occupy at RTP?

Answer: Volume = 0.5 × 24 = 12 dm³

Plenary (5 minutes)

Explain Back

Your student teaches the key points back to you without looking. Fill any gaps immediately.

Lesson 3: Application: gas calculations

Duration: 50 minutes

Starter Activity (5 minutes)

Quick Recall

Recall the key terms: Molar gas volume, Room temperature and pressure (RTP). Define each in one sentence.

Main Content (35 minutes)

Parent/Teacher Guide: Let your student attempt each question alone first, then compare with the model answer. Award method marks for correct working even if the final answer is wrong.

Q1: Higher What volume does 0.5 mol of chlorine gas occupy at RTP?

Answer: Volume = 0.5 × 24 = 12 dm³

Q2: Higher How many moles are in 120 dm³ of neon gas at RTP?

Answer: Moles = 120 ÷ 24 = 5 mol

Q3: Higher What mass of CO₂ (M r = 44) occupies 12 dm³ at RTP?

Answer: Moles = 12 ÷ 24 = 0.5 mol. Mass = 0.5 × 44 = 22 g

Q4: Higher In the reaction 2H₂(g) + O₂(g) → 2H₂O(g), what volume of oxygen is needed to react with 48 dm³ of hydrogen at RTP?

Answer: Mole ratio H₂ : O₂ = 2 : 1. Volume of O₂ = 48 ÷ 2 = 24 dm³

Q5: Higher 4 g of methane (CH₄, M r = 16) is burned completely. Calculate the volume of CO₂ gas produced at RTP.

Answer: Moles of CH₄ = 4 ÷ 16 = 0.25 mol. From the equation, 1 mol CH₄ → 1 mol CO₂, so moles of CO₂ = 0.25 mol. Volume of CO₂ = 0.25 × 24 = 6 dm³

Plenary (5 minutes)

Error Review

Review any questions answered incorrectly. Identify whether the error was knowledge, method, or reading the question.

Lesson 4: Exam Practice: gas calculations

Duration: 50 minutes

Starter Activity (5 minutes)

Command Words

Review what these command words require: state (one point), describe (say what happens), explain (say why), compare (both sides), evaluate (judgement).

Main Content (35 minutes)

Extended Answer

Extended question: Extended Answer 6 marks: Explain how to calculate gas volumes in reactions. <div class="

To calculate gas volumes, first write the balanced symbol equation and identify the mole ratio of gases. At RTP, 1 mole of any gas occupies 24 dm³, so the mole ratio equals the volume ratio. If you know the mass of a reactant, calculate moles (moles = mass ÷ M r ), then use the mole ratio to find moles of the gas, and multiply by 24 to find the volume in dm³. If you know the volume of a gas, calculate moles (moles = volume ÷ 24), use the mole ratio to find moles of the other gas, then convert back to volume. Only calculate gas volumes for gaseous substances — check state symbols carefully. Mark scheme: 1 mark for balanced equation with mole ratio; 1 mark for 24 dm³ per mole at RTP; 1 mark for mole ratio = volume ratio; 1 mark for mass → moles → volume method; 1 mark for volume → moles method; 1 mark for checking state symbols.

Exam Tips: This topic is Higher tier only — it will not appear on Foundation papers | Remember the magic number: 24 dm³ per mole of gas at RTP | Gas volume ratios are the same as mole ratios — this is a shortcut for balanced equation questions | Only use 24 dm³ for gases — do not use it for solids or liquids | Watch the state symbols: H₂O(l) is liquid, H₂O(g) is gas — only calculate gas volumes for substances in the gas state | If a question gives a volume in cm³, convert to dm³ first (÷ 1000) or use 24 000 cm³
Common Errors: Watch Out! Gas volume at RTP depends on the type of gas. Wrong: gas volume depends on gas type at RTP Correct: at RTP, 1 mole of ANY gas occupies 24 dm³ — the particles are far apart so particle size does not matter
AO3 - Reasoning & Interpretation: Analysis and Evaluation A student burns 0.8 g of methane (CH₄, M r = 16) and collects the CO₂ gas produced. Equation: CH₄ + 2O₂ → CO₂ + 2H₂O(l). The student measures 1.0 dm³ of CO₂. Question: Calculate the expected volume of CO₂ at RTP. Comment on the student's result. Answer: Moles of CH₄ = 0.8/16 = 0.05 mol. Ratio 1:1 so moles of CO₂ = 0.05 mol. Expected volume = 0.05 × 24 = 1.2 dm³. The student collected only 1.0 dm³, which is less than expected. Possible reasons: some CO₂ may have dissolved in water, some gas may have escaped during collection, or the reaction may not have gone to completion.
Stretch & Challenge (Grade 8-9):
  • Synoptic links: explain how gas calculations connects to another Combined Science (Trilogy) topic you have studied
  • Real-world: research one real-world use or example of gas calculations
  • Critical: "What are the limitations of the models used in gas calculations?"

Plenary (5 minutes)

Assessment Criteria
  • Got it: Confident explanation + correct worked examples
  • Getting there: Main points OK, needs support with detail
  • Not yet: Confused on key concepts - re-run Lesson 2

Homework & Consolidation

Recommended Resources

🎓 Smart Lesson (Guided)