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c9 chemical 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: c9 chemical calculations

Duration: 50 minutes

Starter Activity (5 minutes)

Quick Recall

Write down everything you already know about c9 chemical calculations. Then check against the key terms: Relative atomic mass (A r ), Relative formula mass (M r ), Mole. Use a mini-whiteboard or paper.

Main Content (35 minutes)

Parent/Teacher Guide:
Before lesson: Read the script below. Pre-teach key vocab: Relative atomic mass (A r ), Relative formula mass (M r ), Mole.
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: c9 chemical calculations. By the end you will be able to answer exam questions on it unaided. It connects to the rest of Chemistry 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: Foundation Calculate the relative formula mass of MgCl₂. (A r : Mg = 24, Cl = 35.5)

Plenary (5 minutes)

Check Out

Your student states one thing they learned and one question they still have about c9 chemical calculations.

Lesson 2: Core Concepts: c9 chemical calculations

Duration: 50 minutes

Starter Activity (5 minutes)

Review Previous Lesson

Quick recap: write 3 key points from Lesson 1 on c9 chemical calculations. Check them against the notes below.

Main Content (35 minutes)

Relative atomic mass (A r ): The average mass of an atom of an element relative to 1/12th the mass of a carbon-12 atom. It is the number on the periodic table.
Relative formula mass (M r ): The sum of all the relative atomic masses of the atoms in a formula. Sometimes called relative molecular mass for molecules.
Mole: The amount of substance containing 6.02 × 10²³ particles (Avogadro's constant). One mole of any substance has a mass in grams equal to its relative formula mass.
Law of conservation of mass: In a chemical reaction, the total mass of the reactants equals the total mass of the products. No atoms are created or destroyed.
Law of conservation of mass: The total mass of reactants equals the total mass of products. Atoms are rearranged but not created or destroyed in a chemical reaction.
State symbols: (s) = solid, (l) = liquid, (g) = gas, (aq) = dissolved in water (aqueous). Always include state symbols when asked.

Practice (10 minutes)

Q: Foundation Calculate the relative formula mass of MgCl₂. (A r : Mg = 24, Cl = 35.5)

Answer: M r (MgCl₂) = 24 + (2 × 35.5) = 24 + 71 = 95

Plenary (5 minutes)

Explain Back

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

Lesson 3: Application: c9 chemical calculations

Duration: 50 minutes

Starter Activity (5 minutes)

Quick Recall

Recall the key terms: Relative atomic mass (A r ), Relative formula mass (M r ), Mole. 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: Foundation Calculate the relative formula mass of MgCl₂. (A r : Mg = 24, Cl = 35.5)

Answer: M r (MgCl₂) = 24 + (2 × 35.5) = 24 + 71 = 95

Q2: Foundation How many moles are there in 20 g of NaOH? (A r : Na = 23, O = 16, H = 1)

Answer: M r (NaOH) = 23 + 16 + 1 = 40. Moles = 20 ÷ 40 = 0.5 mol

Q3: Higher In the reaction Fe₂O₃ + 3CO → 2Fe + 3CO₂, calculate the mass of iron produced from 160 g of Fe₂O₃. (A r : Fe = 56, O = 16, C = 12)

Answer: M r (Fe₂O₃) = (2 × 56) + (3 × 16) = 112 + 48 = 160. Moles of Fe₂O₃ = 160 ÷ 160 = 1 mol. From the equation, 1 mol Fe₂O₃ → 2 mol Fe. Moles of Fe = 2 mol. Mass of Fe = 2 × 56 = 112 g

Q4: Foundation Balance the equation: Al + O₂ → Al₂O₃

Answer: 4Al + 3O₂ → 2Al₂O₃ . Check: Left: Al=4, O=6. Right: Al=4, O=6. ✓

Q5: Higher 13 g of zinc reacts with excess hydrochloric acid. The equation is Zn + 2HCl → ZnCl₂ + H₂. Calculate the mass of zinc chloride produced. (A r : Zn = 65, Cl = 35.5)

Answer: Moles of Zn = 13 ÷ 65 = 0.2 mol. From the equation, 1 mol Zn → 1 mol ZnCl₂, so moles of ZnCl₂ = 0.2 mol. M r (ZnCl₂) = 65 + (2 × 35.5) = 136. Mass of ZnCl₂ = 0.2 × 136 = 27.2 g

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: c9 chemical 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 the mass of product from a balanced symbol equation. <div class="

First, write the balanced symbol equation and identify the mole ratio between the known reactant and the desired product. Then calculate the M r of the known substance and use it to find the number of moles: moles = mass ÷ M r . Next, use the mole ratio from the balanced equation to calculate the moles of the product. Finally, multiply the moles of product by its M r to find the mass: mass = moles × M r . Always show working and include units. Mark scheme: 1 mark for balanced equation with mole ratio; 1 mark for calculating M r ; 1 mark for moles = mass/M r ; 1 mark for using mole ratio; 1 mark for mass = moles × M r ; 1 mark for clear explanation of the sequence.

Exam Tips: Always show your working in calculation questions — you may get marks for method even if the final answer is wrong | When calculating M r , watch out for brackets — the subscript outside a bracket multiplies everything inside it | Use the correct units: g for mass, mol for moles | In reacting mass calculations, always find the moles first, then use the mole ratio from the balanced equation | Check your balanced equations by counting atoms on each side before doing any calculations | If a gas is produced in a reaction in an open container, the mass appears to decrease — this does not break the law of conservation of mass
Common Errors: Watch Out! 1 mole of any substance has a mass of 1 gram. Wrong: 1 mole = 1 gram Correct: 1 mole has a mass in grams equal to the substance's M r — 1 mole of H₂O is 18 g, 1 mole of CO₂ is 44 g Mass is not conserved when a gas is produced in a reaction. Wrong: mass is not conserved when gas is produced Correct: mass is always conserved — if a gas escapes from an open container the total mass appears to decrease, but the mass of gas still counts
AO3 - Reasoning & Interpretation: Analysis and Evaluation In the reaction 2Fe₂O₃ + 3C → 4Fe + 3CO₂, a student uses 32 g of Fe₂O₃ and 6 g of carbon. M r (Fe₂O₃) = 160, A r (C) = 12. Question: Which reactant is in excess? Show your working and explain what this means. Answer: Moles of Fe₂O₃ = 32/160 = 0.2 mol. Moles of C = 6/12 = 0.5 mol. Ratio requires 2 mol Fe₂O₃ : 3 mol C, so for 0.2 mol Fe₂O₃ need 0.3 mol C. Only 0.3 mol C is needed but 0.5 mol is available, so carbon is in excess. Fe₂O₃ is the limiting reactant — it will run out first and determine how much iron is produced.
Stretch & Challenge (Grade 8-9):
  • Synoptic links: explain how c9 chemical calculations connects to another Chemistry topic you have studied
  • Real-world: research one real-world use or example of c9 chemical calculations
  • Critical: "What are the limitations of the models used in c9 chemical 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

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