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Science lesson plan · 12th grade (Year 13 in England) · 30 minutes

Find empirical and molecular formulae

Chemical reactions, acids and bases

CCSS NGSS.HS-PS1-7England: GCSE Chemistry: Quantitative chemistryEngland: A level Chemistry: Amount of substance

Objective and success criteria

Students will: Find empirical and molecular formulae.

  • 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: Percentage by mass of an element in a compound; Moles and reacting masses

Leads to: the next skill in this area.

Materials: Teacher demo or video, Indicator color chart, Mini whiteboards, A diagram or short video clip of the idea, Printed student sheet (below).

Warm-up · 4 min

  • Quick retrieval from earlier learning (Percentage by mass of an element in a compound; Moles and reacting masses). Students answer on mini whiteboards.
  • Show answers together and note who needs support in the main task.
  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. How many moles are in 29.25 g of sodium chloride (NaCl, Mr = 58.5)? → 0.5 molPrompt: moles = mass ÷ Mr
  3. What mass of magnesium is in 20 g of MgO? (Ar: Mg = 24, O = 16) → 12 gPrompt: Find what fraction of the compound's mass is the element.

Hook · 3 min

  • Drop an antacid tablet in water (or show a clip) and ask whether a new substance is being made.
  • Share the objective: Find empirical and molecular formulae. Read the success criteria together.

I do: model it · 7 min

  • Work through the example out loud, saying why you do each step, not just what you do.
  • Example: Find the empirical formula: 11.2 g of iron combines with 4.8 g of oxygen (Ar: Fe = 56, O = 16).
  • Step 1: Fe 0.2 : O 0.3 = 2 : 3
  • Step 2: So the answer is: Fe₂O₃
  • Answer: Fe₂O₃
  • Pause after each step and ask a student to say what comes next.

We do: guided practice · 6 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. Find the empirical formula: 11.2 g of iron combines with 4.8 g of oxygen (Ar: Fe = 56, O = 16). (FeO / Fe₂O₃ / Fe₃O₄ / FeO₃) → Fe₂O₃Prompt: Divide each mass (or percentage) by its Ar to get moles.
  2. A compound has the empirical formula CH₂ (mass of one unit = 14) and Mr = 84. How many empirical units are in one molecule? → 6Prompt: Divide Mr by the mass of one empirical unit.

You do: independent practice · 7 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 6 questions are on the student sheet below, with answers in the key.

Exit ticket · 3 min

  • Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.
  1. Find the empirical formula: a hydrocarbon is 80 % carbon and 20 % hydrogen by mass (Ar: C = 12, H = 1). (CH₄ / C₂H₆ / CH₃ / C₄H) → CH₃Prompt: Divide each mass (or percentage) by its Ar to get moles.
  2. A compound has the empirical formula CH₂ (mass of one unit = 14) and Mr = 84. How many empirical units are in one molecule? → 6Prompt: Divide Mr by the mass of one empirical unit.

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: Percentage by mass of an element in a compound.
  • Use concrete materials: teacher demo or video.
  • Give the first step of each question, or a partly worked example to finish.
  • Pair with a confident partner for the guided practice.

Stretch

  • 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-12.

Free lesson plan from eurekami.com/lesson-plan-builder

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Find empirical and molecular formulae

Name

Date

Score/ 6

  1. Question 1

    Find the empirical formula: a hydrocarbon is 75 % carbon and 25 % hydrogen by mass (Ar: C = 12, H = 1).

    • A. CH₄
    • B. C₃H
    • C. C₂H₆
    • D. CH₃

  2. Question 2

    A compound has the empirical formula CH₂ (mass of one unit = 14) and Mr = 84. How many empirical units are in one molecule?

    Answer

  3. Question 3

    Find the empirical formula: a compound is 40 % carbon, 6.7 % hydrogen and 53.3 % oxygen (Ar: C = 12, H = 1, O = 16).

    • A. CH₂O
    • B. C₂H₄O
    • C. C₆H₁₂O₆
    • D. CHO

  4. Question 4

    A compound has the empirical formula C₂H₅ (mass of one unit = 29) and Mr = 58. How many empirical units are in one molecule?

    Answer

  5. Question 5

    A compound has the empirical formula CH₂ (mass of one unit = 14) and Mr = 56. How many empirical units are in one molecule?

    Answer

  6. Question 6

    A compound has the empirical formula CH₂O (mass of one unit = 30) and Mr = 180. How many empirical units are in one molecule?

    Answer

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Answer key: Find empirical and molecular formulae

  1. 1. A. CH₄

  2. 2. 6

  3. 3. A. CH₂O

  4. 4. 2

  5. 5. 4

  6. 6. 6