Science lesson plan · 12th grade (Year 13 in England) · 60 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 · 6 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.
- What is the percentage by mass of nitrogen in ammonium nitrate, NH₄NO₃? (Ar: N = 14, H = 1, O = 16) Give 1 decimal place. → 35 %Prompt: Work out Mr of the compound first.
- How many moles are in 20 g of calcium carbonate (CaCO₃, Mr = 100)? → 0.2 molPrompt: moles = mass ÷ Mr
- What mass of nitrogen is in 40 g of NH₄NO₃? (Ar: N = 14, H = 1, O = 16) → 14 gPrompt: Find what fraction of the compound's mass is the element.
Hook · 6 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 · 12 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 · 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.
- Find the empirical formula: a compound is 40 % carbon, 6.7 % hydrogen and 53.3 % oxygen (Ar: C = 12, H = 1, O = 16). (CHO / C₂H₄O / C₆H₁₂O₆ / CH₂O) → CH₂OPrompt: Divide each mass (or percentage) by its Ar to get moles.
- A compound has the empirical formula CH₂O (mass of one unit = 30) and Mr = 180. How many empirical units are in one molecule? → 6Prompt: Divide Mr by the mass of one empirical unit.
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.
- A compound has the empirical formula NO₂ (mass of one unit = 46) and Mr = 92. How many empirical units are in one molecule? → 2Prompt: Divide Mr by the mass of one empirical unit.
- Find the empirical formula: 11.2 g of iron combines with 4.8 g of oxygen (Ar: Fe = 56, O = 16). (Fe₂O₃ / Fe₃O₄ / FeO₃ / FeO) → Fe₂O₃Prompt: Divide each mass (or percentage) by its Ar to get moles.
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