• Quick retrieval from earlier learning (Concentration in mol/dm³; Neutralisation and naming salts). Students answer on mini whiteboards.
• Show answers together and note who needs support in the main task.

1. 0.2 mol of hydrochloric acid is in 100 cm³ of solution. What is the concentration, in mol/dm³? → 2 mol/dm³Prompt: concentration = moles ÷ volume in dm³
2. Name the salt: magnesium + nitric acid → ___ + hydrogen (magnesium nitrate / magnesium oxide / magnesium chloride / magnesium sulfate) → magnesium nitratePrompt: The first part of the salt's name comes from the metal.
3. How many moles of sodium hydroxide are in 50 cm³ of a 0.2 mol/dm³ solution? → 0.01 molPrompt: moles = concentration × volume in dm³

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

• Work through the example out loud, saying why you do each step, not just what you do.
• Example: 20 cm³ of 0.5 mol/dm³ sodium hydroxide is neutralised by 12.5 cm³ of hydrochloric acid. HCl + NaOH → NaCl + H₂O. What is the concentration of the acid, in mol/dm³? Give 3 decimal places.
• Step 1: n(NaOH) = 0.5 × 0.02 = 0.01
• Step 2: n(HCl) = 0.01
• Step 3: 0.01 ÷ 0.0125 = 0.8
• Step 4: So the answer is: 0.8 mol/dm³
• Answer: 0.8 mol/dm³
• Pause after each step and ask a student to say what comes next.

• Do these together. Students try each step on whiteboards before you reveal it.
• Use the hints as prompts rather than giving the answer.

1. 20 cm³ of 0.2 mol/dm³ sodium hydroxide is neutralised by 40 cm³ of hydrochloric acid. HCl + NaOH → NaCl + H₂O. What is the concentration of the acid, in mol/dm³? Give 3 decimal places. → 0.1 mol/dm³Prompt: Find moles of sodium hydroxide: concentration × volume in dm³.
2. Titres (cm³): rough 22.37, run 1 21.57, run 2 21.67. Ignoring the rough titre, calculate the mean titre from the concordant results (within 0.10 cm³), in cm³. Give 2 decimal places. → 21.62 cm³Prompt: The rough titre is only a guide.

• 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.

• Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.

1. Titres (cm³): rough 24.79, run 1 24.19, run 2 24.24. Ignoring the rough titre, calculate the mean titre from the concordant results (within 0.10 cm³), in cm³. Give 2 decimal places. → 24.22 cm³Prompt: The rough titre is only a guide.
2. Why is the conical flask swirled during a titration? (to cool the flask / to change the indicator / to speed up evaporation / to mix the solutions so the end point is seen accurately) → to mix the solutions so the end point is seen accuratelyPrompt: The colour should change evenly.

Free online practice with hints and printable worksheets for this skill: eurekami.com/science/grade-12.