Educerie · IB Diploma · Chemistry
Structure 1 — Topic test · mark scheme
One mark per distinct point. Accept alternative wording unless a term is required, shown in bold. Award error-carried-forward (ECF) throughout: a wrong value used correctly in a later part still earns that part's marks.
1Isotopes and relative atomic mass — 5 marks
(a) [1]
| Atoms of the same element with the same number of protons but different numbers of neutrons | 1 |
|---|
Both halves required. "Atoms with different mass numbers" alone scores 0 — it does not establish that they are the same element.
(b) [2]
Correct expression, e.g. (63 × 69.2 + 65 × 30.8) / 100 | 1 |
|---|---|
63.6 | 1 |
The abundances are percentages here, so dividing by 100 is correct. Award the second mark only for an answer to 3 s.f. as instructed;
63.616unrounded scores 1. A value outside the range 63–65 indicates a method error and scores 0 for the second mark even with working shown.
(c) [2]
| Chemical properties are determined by the electron configuration / number of outer electrons | 1 |
|---|---|
| Isotopes have the same number of protons and therefore the same number of electrons; the neutrons do not affect bonding | 1 |
An answer that says only "they have the same number of electrons" without linking electrons to chemical behaviour scores 1.
2Electron configuration and ionisation energy — 5 marks
(a) [2]
Correct configuration up to argon core, i.e. 1s² 2s² 2p⁶ 3s² 3p⁶ | 1 |
|---|---|
3d⁶ with no 4s electrons | 1 |
Full answer:
1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶. Accept[Ar] 3d⁶.[Ar] 3d⁴ 4s²is the expected error — 4s electrons are removed first on ionisation — and scores 1 for the core only.
(b) [3]
| Identifies the large increase between the second and third ionisation energies | 1 |
|---|---|
| Explains that the third electron is removed from a shell closer to the nucleus / with less shielding / a full inner shell | 1 |
| Concludes group 2 | 1 |
Deduce requires the reasoning. "Group 2" stated alone scores 1 of 3. Accept ratio or difference arguments for the first mark provided the location of the jump is explicit.
3Stoichiometry and gas volume — 6 marks
(a) [1]
n = 0.240 / 24.31 = 9.87 × 10⁻³ mol | 1 |
|---|
Accept
9.9 × 10⁻³or0.00987.
(b) [1]
n = 0.500 × 0.0500 = 0.0250 mol | 1 |
|---|
The volume must be converted to dm³. Using 50.0 gives 25.0 mol and scores 0.
(c) [2]
Divides each amount by its coefficient, e.g. Mg: 9.87 × 10⁻³ ÷ 1 and HCl: 0.0250 ÷ 2 = 0.0125 | 1 |
|---|---|
| Concludes magnesium is limiting | 1 |
The first mark is specifically for accounting for the 2:1 ratio. Comparing the raw amounts
9.87 × 10⁻³ < 0.0250and concluding Mg reaches the right answer by the wrong route — award the second mark only, for a maximum of 1.
(d) [2]
Rearranges and substitutes correctly, e.g. V = (9.87 × 10⁻³ × 8.31 × 273) / (1.00 × 10⁵) | 1 ECF from (c) |
|---|---|
0.224 dm³ (accept 2.24 × 10⁻⁴ m³ or 224 cm³) | 1 |
The answer must be converted to dm³ as the question requires;
2.24 × 10⁻⁴with no unit scores 0 for the second mark. An answer computed with 273 °C rather than K scores the method mark only.
4Ideal gases — 4 marks
(a) [2 max] — any two of:
| Particles have negligible volume compared with the container | 1 |
|---|---|
| There are no intermolecular forces between particles | 1 |
| Collisions are perfectly elastic / no kinetic energy is lost | 1 |
| Particles are in constant random motion | 1 |
(b) [2]
| At low temperature the particles have less kinetic energy and move more slowly | 1 |
|---|---|
| So intermolecular attractions become significant relative to that energy, the particles are pulled together, and the gas no longer behaves ideally (approaching condensation) | 1 |
The second mark requires the assumption being broken to be named. "The particles slow down" alone scores 1.
Mark distribution
| Split | Allocation |
|---|---|
| By sub-topic | S1.2 Nuclear atom 5 · S1.3 Electron configurations 5 · S1.4 The mole 6 · S1.5 Ideal gases 4 |
| By objective | AO1 knowledge 5 · AO2 application 8 · AO3 analysis and evaluation 7 |
| By command term | Calculate 6 · Explain 6 · Deduce 5 · State 2 · Define 1 |
Total: 20 marks
Educerie · original mark scheme for original questions, written against the published IB syllabus structure for Chemistry Structure 1, first assessment 2025. Last reviewed 5 September 2026.