The shape of the lesson
By the end of the lesson, learners can
- A chemist's dozen: Extended content.
- Mass, moles, particles: Work every conversion through the mole: divide mass by M r to get moles; multiply moles by 6.02 × 10 23 to get particles.
- The volume of a gas: Switch to the gas volumes tab.
- moles = mass ÷ Mr with the mass in grams.
- particles = moles × 6.02 × 1023; say which particle: atoms or molecules.
- Gas moles = volume in dm3 ÷ 24 (or cm3 ÷ 24 000) at r.t.p.
- Gases only for the 24 dm3 rule.
One picture every idea returns to
One mole of a substance contains 6.02 × 1023 particles (the Avogadro constant). The mass of one mole in grams equals its Ar or Mr, so moles = mass ÷ Mr. One mole of any gas occupies 24 dm3 at room temperature and pressure (r.t.p.), so moles of gas = volume in dm3 ÷ 24, or volume in cm3 ÷ 24 000.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Counting by weighing.. Nobody can count 10 23 atoms, but anyone can weigh them. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: A chemist's dozen | Extended content. Project tab 2 of the lab (Stoichiometry lab, the mole) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Mass, moles, particles | Work every conversion through the mole: divide mass by M r to get moles; multiply moles by 6.02 × 10 23 to get particles. | Pairs |
| 21 to 30 min | Teach and check: The volume of a gas | Switch to the gas volumes tab. Project tab 5 of the lab (Stoichiometry lab, gas volumes) and let a student drive it while the class predicts. | Round Table |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 13 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in The volume of a gas, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Round Table
One sheet travels round the team. Each student adds one line of working or one idea and passes it on, so the solution is built four hands at a time and every contribution is visible.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Read whether the question wants molecules or atoms. 9 g of water is 3.01 × 10 23 molecules but 9.03 × 10 23 atoms, because each H 2 O molecule contains three atoms. | Return to Mass, moles, particles: ask a pair to state the correct version and show it on the lab before moving on. |
| The 24 dm 3 rule is for gases only, at r.t.p. | Never use it for a solid, a liquid or a solution. And check the unit: dividing a volume in cm 3 by 24 gives an answer 1000 times too big. |
The mole and gas volumes: worksheet
6 questions, 13 marks. Written in the style of the papers; no past paper question is reproduced.
State the number of particles in one mole of a substance.
For 22 g of carbon dioxide, CO2, calculate (a) the number of moles, (b) the number of molecules and (c) its volume at r.t.p. (Ar: C 12, O 16)
Calculate the volume, in cm3, occupied by 0.16 g of oxygen gas, O2, at r.t.p. (Ar: O 16)
Calculate the mass of 600 cm3 of carbon dioxide at r.t.p. (Ar: C 12, O 16)
A student has 4 g of hydrogen gas, H2, and 32 g of oxygen gas, O2. Which sample contains more molecules, and which has the larger volume at r.t.p.? Show your working. (Ar: H 1, O 16)
Calculate the mass of 3.01 × 1022 atoms of zinc. (Ar: Zn 65)
Answer key and marking notes
6.02 × 1023 particles (the Avogadro constant).
- (a) Mr = 44, so moles = 22 ÷ 44 = 0.5 mol.
- (b) 0.5 × 6.02 × 1023 = 3.01 × 1023 molecules.
- (c) 0.5 × 24 = 12 dm3.
- Mr of O2 = 32, so moles = 0.16 ÷ 32 = 0.005 mol.
- Volume = 0.005 × 24 000 = 120 cm3.
- Moles = 600 ÷ 24 000 = 0.025 mol.
- Mass = 0.025 × 44 = 1.1 g.
- Hydrogen: 4 ÷ 2 = 2 mol. Oxygen: 32 ÷ 32 = 1 mol.
- Hydrogen has twice as many molecules (1.20 × 1024 against 6.02 × 1023).
- Hydrogen also has the larger volume: 2 × 24 = 48 dm3 against 24 dm3.
- Moles = 3.01 × 1022 ÷ 6.02 × 1023 = 0.05 mol.
- Mass = 0.05 × 65 = 3.25 g.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
What is the mass of 0.25 mol of calcium hydroxide, Ca(OH)2? (Mr 74)
- A18.5 g
- B296 g
- C0.0034 g
- D10 g
How many oxygen atoms are in 0.2 mol of carbon dioxide, CO2? (Avogadro constant 6.02 × 1023 per mol)
- A2.41 × 1023
- B1.20 × 1023
- C3.61 × 1023
- D2.41 × 1022
What volume does 3.2 g of methane, CH4, occupy at r.t.p.? (Ar: H 1, C 12)
- A4.8 dm3
- B76.8 dm3
- C0.13 dm3
- D1.5 dm3
Which sample contains the greatest number of molecules? (Ar: H 1, C 12, N 14, O 16, S 32)
- A36 g of water, H2O
- B96 g of sulfur dioxide, SO2
- C66 g of carbon dioxide, CO2
- D28 g of nitrogen, N2
How many moles of gas are in 360 cm3 at r.t.p.?
- A0.015 mol
- B15 mol
- C0.15 mol
- D8640 mol
Which statement about 1 mol of oxygen, O2, and 1 mol of carbon dioxide, CO2, both at r.t.p., is correct?
- AThey have the same volume and the same number of molecules, but different masses
- BThey have the same mass
- CThe carbon dioxide has the larger volume because its molecules are heavier
- DThey contain the same number of atoms
What is the mass of 1.204 × 1024 molecules of water, H2O? (Mr 18; Avogadro constant 6.02 × 1023 per mol)
- A36 g
- B9 g
- C18 g
- D2 g
2.0 g of a gas occupies 1.5 dm3 at r.t.p. What is its relative molecular mass?
- A32
- B1.33
- C48
- D18
Quiz key
mass = moles × Mr = 0.25 × 74 = 18.5 g. 296 g divides 74 by 0.25; 0.0034 g divides 0.25 by 74; 10 g uses only the calcium (0.25 × 40).
Each CO2 has 2 O atoms, so 0.2 mol of CO2 contains 0.4 mol of O atoms. 0.4 × 6.02 × 1023 = 2.41 × 1023. 1.20 × 1023 is the number of molecules; 3.61 × 1023 counts all three atoms in each molecule (0.6 mol); 2.41 × 1022 has the power of ten wrong.
Mr(CH4) = 16, so moles = 3.2 ÷ 16 = 0.2 mol. Volume = 0.2 × 24 = 4.8 dm3. 76.8 multiplies the mass by 24 without converting to moles; 0.13 divides the mass by 24; 1.5 divides 24 by 16.
Water: 36 ÷ 18 = 2 mol. SO2: 96 ÷ 64 = 1.5 mol. CO2: 66 ÷ 44 = 1.5 mol. N2: 28 ÷ 28 = 1 mol. Water has the most moles, so the most molecules, even though sulfur dioxide has the largest mass.
1 mol of gas occupies 24 dm3 = 24 000 cm3. moles = 360 ÷ 24 000 = 0.015 mol. 15 mol divides the cm3 value by 24, giving an answer 1000 times too big; 8640 multiplies by 24 instead of dividing.
One mole of any gas contains 6.02 × 1023 molecules and occupies 24 dm3 at r.t.p., whatever the gas. The masses differ (32 g against 44 g). Heavier molecules do not take up more volume, and CO2 has three atoms per molecule while O2 has two, so the numbers of atoms differ.
moles = 1.204 × 1024 ÷ 6.02 × 1023 = 2 mol. mass = 2 × 18 = 36 g. 9 g divides 18 by 2; 18 g is the mass of only 1 mol; 2 g gives the number of moles as if it were a mass.
moles = 1.5 ÷ 24 = 0.0625 mol. Mr = 2.0 ÷ 0.0625 = 32 (the gas could be oxygen). 1.33 divides the mass by the volume; 48 multiplies the mass by 24; 18 is (1.5 × 24) ÷ 2.0, the calculation turned upside down.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.