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DSE Chemistry: The Mole and Concentration — Tutor Lesson Plan
A student reads “4.0 g” and writes “4.0 mol”. Every later mark falls with it, so this lesson makes conversion to moles the first move of every calculation.
Objectives — By the end students can: convert mass to moles with n = m divided by M; find concentration in mol per cubic decimetre; and work out percentage yield from theoretical and actual amounts.
01 Prerequisites (3-minute check)
If any of these fails, fix it before the lesson starts.
| Skill | Quick check | Pass mark |
|---|---|---|
| Molar mass | What is the molar mass of sodium hydroxide | 40.0 g per mol |
| Volume units | Convert 250 cubic centimetres to cubic decimetres | 0.250 |
| Significant figures | How many figures does 4.0 have | two |
02 Lesson flow (60 min)
| Time | Stage | Tutor does | Student does |
|---|---|---|---|
| 0-8 min | What a mole is | A counting unit, not a mass | Say how many particles |
| 8-20 min | Mass to moles | n = m divided by M | 4.0 g gives 0.10 mol |
| 20-34 min | Concentration | c = n divided by V, with V in cubic decimetres | 0.40 mol per cubic decimetre |
| 34-46 min | Gas volume | 24 cubic decimetres per mole at room conditions | 0.15 mol gives 3.6 |
| 46-56 min | Percentage yield | actual over theoretical times 100 | 80.0 percent |
| 56-60 min | Exit check | Three questions collected and marked | Work alone |
03 Teaching script (read it out)
Scene 1: A mole counts particles, it is not a mass
Sodium hydroxide has a molar mass of 40.0 g per mol, so 4.0 g is 0.10 mol.
The recurring error is to carry 4.0 straight through as moles, which wrecks the concentration that follows.
The recurring error is to carry 4.0 straight through as moles, which wrecks the concentration that follows.
Student states how many moles 8.0 g of sodium hydroxide is (0.20) and why not 8.0.
Scene 2: Concentration only works in cubic decimetres
0.10 mol in 250 cubic centimetres is 0.10 divided by 0.250, which is 0.40 mol per cubic decimetre.
Dividing by 250 instead gives a value a thousand times too small, which is a useful sanity signal.
Dividing by 250 instead gives a value a thousand times too small, which is a useful sanity signal.
Note: the unit mol per cubic decimetre goes in the answer; a bare M is discouraged in DSE scripts.
Scene 3: Yield compares like with like
A theoretical 0.20 mol against 0.16 mol obtained gives 80.0 percent.
If the two amounts refer to different substances, convert them with the equation coefficients before dividing.
If the two amounts refer to different substances, convert them with the equation coefficients before dividing.
Student calculates the yield for 0.25 mol theoretical and 0.20 mol obtained (80.0 percent).
04 Question types and worked steps
| Type | Example | Steps | Answer |
|---|---|---|---|
| Moles | 4.0 g, molar mass 40.0 | n = m divided by M | 0.10 mol |
| Concentration | 0.10 mol in 250 cubic centimetres | Convert the volume first | 0.40 mol per cubic decimetre |
| Gas volume | 0.15 mol at room conditions | Multiply by 24 | 3.6 cubic decimetres |
| Percentage yield | theoretical 0.20, actual 0.16 | Actual over theoretical times 100 | 80.0 percent |
Sanity check (teach this): A concentration sits between roughly 0.01 and 5 mol per cubic decimetre. A value like 400 on a dilute solution means cubic centimetres were treated as cubic decimetres.
05 Independent practice (3 questions)
| Question | Answer | What to watch |
|---|---|---|
| Moles in 4.0 g of sodium hydroxide | 0.10 mol | Molar mass used, not the mass |
| Concentration of 0.10 mol in 250 cubic centimetres | 0.40 mol per cubic decimetre | Volume converted |
| Yield from 0.20 mol theoretical and 0.16 mol actual | 80.0 percent | Same substance on both sides |
06 Common mistakes and what to say
| Mistake | Student writes | Say this |
|---|---|---|
| Mass used as moles | 4.0 g written as 4.0 mol | Grams must be divided by molar mass first. |
| Volume left in cubic centimetres | a concentration of 400 | Convert to cubic decimetres, which means dividing by 1000. |
| Coefficients ignored | a yield of 40 percent | Match both amounts to the same substance with the equation. |
| Significant figures inflated | 0.40 written as 0.4000 | Two figures in the question means two in the answer. |
07 Homework (paper, section and question numbers)
- DSE Chemistry practice paper (moles and concentration) — Section A, questions 4 to 6
- Secondary chemistry chapter index — All chapters
08 Exit check (last 5 minutes)
| Question | Answer | Decision |
|---|---|---|
| Moles in 4.0 g of sodium hydroxide | 0.10 mol | Three right: move to acids and bases |
| Concentration of 0.10 mol in 250 cubic centimetres | 0.40 mol per cubic decimetre | One wrong: re-teach units |
| Yield from 0.20 and 0.16 mol | 80.0 percent | Two wrong: repeat scene 3 |
09 Tutor FAQ
Must students memorise molar masses?
Common ones, yes; otherwise the paper supplies them or the periodic table does.
May concentration be given in grams per cubic decimetre?
Yes when stated, but mole ratios need moles.
Can yield exceed 100 percent?
Not in principle; above 100 percent means wet or impure product, which is an error to explain.
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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.
