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Science lesson plan · 11th grade (Year 12 in England) · 60 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 · 6 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.
  1. 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.
  2. Calculate the relative formula mass (Mr) of calcium carbonate, CaCO₃. Relative atomic masses: Ca = 40, C = 12, O = 16. → 100Prompt: Count the atoms of each element first, including any brackets.
  3. What is the percentage by mass of carbon in C₂H₅OH? Relative atomic masses: C = 12, H = 1, O = 16. Give 1 decimal place. → 52.2 %Prompt: Work out Mr first.

Hook · 6 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 · 12 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 · 12 min

  • Do these together. Students try each step on whiteboards before you reveal it.
  • Use the hints as prompts rather than giving the answer.
  1. What is the percentage by mass of carbon in carbon dioxide, CO₂? (Ar: C = 12, O = 16) Give 1 decimal place. → 27.3 %Prompt: Work out Mr of the compound first.
  2. What mass of carbon is in 900 g of C₆H₁₂O₆? (Ar: C = 12, H = 1, O = 16) → 360 gPrompt: Find what fraction of the compound's mass is the element.

You do: independent practice · 17 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 · 7 min

  • Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.
  1. What is the percentage by mass of magnesium in magnesium oxide, MgO? (Ar: Mg = 24, O = 16) Give 1 decimal place. → 60 %Prompt: Work out Mr of the compound first.
  2. Which iron ore gives more iron per tonne: Fe₂O₃ (70 % Fe) or Fe₃O₄ (72 % Fe)? (Fe₂O₃ / neither contains iron / Fe₃O₄ / they give the same) → Fe₃O₄Prompt: Compare the percentages.

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

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Percentage by mass of an element in a compound

Name

Date

Score/ 8

  1. Question 1

    What mass of calcium is in 200 g of CaCO₃? (Ar: Ca = 40, C = 12, O = 16)

    Answerg

  2. Question 2

    What is the percentage by mass of sulfur in sulfuric acid, H₂SO₄? (Ar: H = 1, S = 32, O = 16) Give 1 decimal place.

    Answer%

  3. Question 3

    Which iron ore gives more iron per tonne: Fe₂O₃ (70 % Fe) or Fe₃O₄ (72 % Fe)?

    • A. Fe₃O₄
    • B. Fe₂O₃
    • C. they give the same
    • D. neither contains iron

  4. Question 4

    What mass of nitrogen is in 800 g of NH₄NO₃? (Ar: N = 14, H = 1, O = 16)

    Answerg

  5. Question 5

    What is the percentage by mass of carbon in carbon dioxide, CO₂? (Ar: C = 12, O = 16) Give 1 decimal place.

    Answer%

  6. Question 6

    What mass of iron is in 800 g of Fe₂O₃? (Ar: Fe = 56, O = 16)

    Answerg

  7. Question 7

    What is the percentage by mass of calcium in calcium carbonate, CaCO₃? (Ar: Ca = 40, C = 12, O = 16) Give 1 decimal place.

    Answer%

  8. Question 8

    Which compound has the higher percentage by mass of oxygen: water (H₂O) or carbon dioxide (CO₂)? (Ar: H = 1, C = 12, O = 16)

    • A. water: 16/18 is about 89 %
    • B. carbon dioxide: 32/44 is about 73 %
    • C. they are the same
    • D. neither contains oxygen

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Answer key: Percentage by mass of an element in a compound

  1. 1. 80 g

  2. 2. 32.7 %

  3. 3. A. Fe₃O₄

  4. 4. 280 g

  5. 5. 27.3 %

  6. 6. 560 g

  7. 7. 40 %

  8. 8. A. water: 16/18 is about 89 %