Science lesson plan · 11th grade (Year 12 in England) · 45 minutes
Percentage by mass of an element in a compound
Atoms, elements and the periodic table
CCSS NGSS.HS-PS1-7England: GCSE Combined Science: Quantitative chemistry
Objective and success criteria
Students will: Percentage by mass of an element in a compound.
- I can answer questions on today's skill on my own, without help.
- I can explain each step and say why it works.
- I can check my answer and spot a mistake.
Before and after
Needs first: Relative formula mass
Leads to: Find empirical and molecular formulae
Materials: Periodic tables, Atom model kits, Mini whiteboards, A diagram or short video clip of the idea, Printed student sheet (below).
Warm-up · 5 min
- Quick retrieval from earlier learning (Relative formula mass). Students answer on mini whiteboards.
- Show answers together and note who needs support in the main task.
- Calculate the relative formula mass (Mr) of NaOH. Relative atomic masses: Na = 23, O = 16, H = 1. → 40Prompt: Multiply each relative atomic mass by the number of those atoms.
- Calculate the relative formula mass (Mr) of magnesium nitrate, Mg(NO₃)₂. Relative atomic masses: Mg = 24, N = 14, O = 16. → 148Prompt: Count the atoms of each element first, including any brackets.
- What is the percentage by mass of aluminium (aluminum) in Al₂O₃? Relative atomic masses: Al = 27, O = 16. Give 1 decimal place. → 52.9 %Prompt: Work out Mr first.
Hook · 5 min
- Show the periodic table and ask what patterns students can see before anything is explained.
- Share the objective: Percentage by mass of an element in a compound. Read the success criteria together.
I do: model it · 10 min
- Work through the example out loud, saying why you do each step, not just what you do.
- Example: What is the percentage by mass of carbon in carbon dioxide, CO₂? (Ar: C = 12, O = 16) Give 1 decimal place.
- Step 1: Mr = 44
- Step 2: 12 ÷ 44 × 100 = 27.27
- Step 3: So the answer is: 27.3 %
- Answer: 27.3 %
- Pause after each step and ask a student to say what comes next.
We do: guided practice · 10 min
- Do these together. Students try each step on whiteboards before you reveal it.
- Use the hints as prompts rather than giving the answer.
- What is the percentage by mass of iron in iron(III) oxide, Fe₂O₃? (Ar: Fe = 56, O = 16) Give 1 decimal place. → 70 %Prompt: Work out Mr of the compound first.
- What mass of magnesium is in 200 g of MgO? (Ar: Mg = 24, O = 16) → 120 gPrompt: Find what fraction of the compound's mass is the element.
You do: independent practice · 10 min
- Students work on the student sheet. Circulate and check the first two answers of every student early.
- Students who finish: write a question of their own for a partner, with the answer.
The 8 questions are on the student sheet below, with answers in the key.
Exit ticket · 5 min
- Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.
- What is the percentage by mass of sulfur in sulfuric acid, H₂SO₄? (Ar: H = 1, S = 32, O = 16) Give 1 decimal place. → 32.7 %Prompt: Work out Mr of the compound first.
- What mass of iron is in 1600 g of Fe₂O₃? (Ar: Fe = 56, O = 16) → 1120 gPrompt: Find what fraction of the compound's mass is the element.
Watch out for
- Everyday meanings of words getting in the way of the science meaning: define key words explicitly.
- Remembering facts without the reason: ask "why?" and "what is the evidence?"
Support and stretch
Support
- Review first: Relative formula mass.
- Use concrete materials: periodic tables.
- Give the first step of each question, or a partly worked example to finish.
- Pair with a confident partner for the guided practice.
Stretch
- Move on to: Find empirical and molecular formulae.
- Explain a common mistake in writing and how to fix it.
- Write a word problem that uses today's skill, then solve a partner's.
Homework and more practice
Free online practice with hints and printable worksheets for this skill: eurekami.com/science/grade-11.
Free lesson plan from eurekami.com/lesson-plan-builder