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S4 Chemistry: Mole and Stoichiometry Paper 1 (Moles, Mass and Particles)
S4 Chemistry: Mole and Stoichiometry Paper 1 (Moles, Mass and Particles)
This paper starts from molar mass and drills mass, mole and particle conversions with 6.02e23 throughout, ending on the classic slip of using 16 for an oxygen molecule.
Quick answer 1. 2 mol | 2. 45 g | 3. 1.204e24 | 4. 29.25 g | 5. 3.01e23 | 6. 96 g | 7. 0.1 mol | 8. 25 g and 4.515e23 oxygen atoms | 9. 280 g, 25 portions | 10. it is 1 mol
01 Paper details (check answers afterwards)
| Item | Detail |
|---|---|
| Questions | 10 questions |
| Suggested time | 25 minutes |
| Difficulty mix | Foundation 4 · Intermediate 3 · Challenge 2 · Error redo 1 |
| Chapter | Chapter: S4 Chemistry – Mole and Stoichiometry |
| Level | Foundation |
02 Questions (10 items)
- FoundationHow many moles are in 24 g of carbon (molar mass 12 g/mol)?
- FoundationWhat is the mass of 2.5 mol of water (H2O, molar mass 18 g/mol)?
- FoundationHow many molecules are in 88 g of carbon dioxide (CO2, molar mass 44 g/mol)?
- FoundationWhat is the mass of 0.5 mol of sodium chloride (NaCl, molar mass 58.5 g/mol)?
- IntermediateHow many water molecules are in 9 g of water (18 g/mol)?
- IntermediateWhat is the mass of 3 mol of oxygen gas (O2, molar mass 32 g/mol)?
- IntermediateA sample contains 6.02e22 particles. How many moles is that?
- ChallengeWhat is the mass of 0.25 mol of calcium carbonate (CaCO3, 100 g/mol), and how many oxygen atoms does it contain?
- ChallengeWhat is the mass of 5 mol of iron (56 g/mol), and how many 0.2 mol portions can it be split into?
- Error redoA student says 32 g of oxygen gas is 2 mol. What is wrong?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | n = 24 / 12 = 2 mol. | Moles equal mass divided by molar mass, with matching units. |
| 2 | m = 2.5 x 18 = 45 g. | Turn moles into mass by multiplying by molar mass. |
| 3 | n = 88 / 44 = 2 mol; particles = 2 x 6.02e23 = 1.204e24. | Find moles first, then multiply by the Avogadro constant. |
| 4 | m = 0.5 x 58.5 = 29.25 g. | Multiply by the decimal molar mass; do not round it yourself. |
| 5 | n = 9 / 18 = 0.5 mol; molecules = 0.5 x 6.02e23 = 3.01e23. | Go mass to moles to particles in three steps. |
| 6 | m = 3 x 32 = 96 g. | O2 is diatomic, so use 32 not 16. |
| 7 | n = 6.02e22 / 6.02e23 = 0.1 mol. | Particles over the constant gives moles; one power of ten is a factor of ten. |
| 8 | m = 0.25 x 100 = 25 g; each CaCO3 has 3 oxygen atoms, so 0.25 x 3 x 6.02e23 = 4.515e23. | Count the atoms inside one formula unit before multiplying. |
| 9 | m = 5 x 56 = 280 g; portions = 5 / 0.2 = 25. | Mass and splitting are separate steps: get moles first. |
| 10 | Wrong: the molar mass of O2 is 32 g/mol, so 32 / 32 = 1 mol; only treating 16 as the molar mass of O2 gives 2 mol. | Oxygen atom is 16, oxygen molecule is 32; mixing them up causes this slip. |
04 What to do after marking
- After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
- Repeat the same item two days later; move to the next paper only when both attempts are correct.
- Parents only need the answer and note columns; there is no need to explain every item.
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