AS Chemistry: Ideal Gases and Structure — Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs — examination boards hold copyright in their own papers. Use these alongside the official past papers available free from your board.

Related: Ideal Gases and Structure revision notes

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Questions

1. State three assumptions of the kinetic theory of an ideal gas. [3]

2. A gas occupies 250 cm³ at 27 °C and 1.20 × 10⁵ Pa. (R = 8.31 J K⁻¹ mol⁻¹)

(a) Convert the volume to m³ and the temperature to K. [2] (b) Calculate the number of moles. [3] (c) The mass of the gas is 0.482 g. Calculate its relative molecular mass. [2]

3. Explain, under what conditions real gases deviate most from ideal behaviour, and why. [4]

4. Compare diamond, graphite and iodine.

(a) State the structure and bonding of each. [3] (b) Explain why graphite conducts electricity but diamond does not. [3] (c) Iodine has a melting point of only 114 °C, and also has a noticeable vapour pressure below that temperature, which is why it appears to sublime. Explain both observations. [2]

5. Explain why sodium chloride conducts electricity when molten but not when solid. [2]

6. Explain why copper conducts electricity in both the solid and molten state. [3]

7. An unknown white solid has a very high melting point, does not conduct electricity as a solid, but conducts well once molten, and dissolves readily in water to give a colourless solution. Deduce its type of structure and bonding, explaining how each observation rules out the alternatives. [5]

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8. Explain why graphite conducts electricity but diamond does not. [3]

Answers

1. Any three: a large number of molecules in random motion [1]; the volume of the molecules is negligible compared with the container [1]; no intermolecular forces except during collisions [1]; collisions are perfectly elastic; collision time is negligible.

2. (a) V = 250 ÷ 10⁶ = 2.50 × 10⁻⁴ m³ [1]; T = 27 + 273 = 300 K [1].

(b) n = pV ÷ RT = (1.20 × 10⁵ × 2.50 × 10⁻⁴) ÷ (8.31 × 300) [1] = 30.0 ÷ 2493 [1] = 0.01203 mol [1].

(c) M = m ÷ n = 0.482 ÷ 0.01203 [1] = 40.1 [1].

3. At high pressure, the molecules are close together so their own volume is no longer negligible compared with the container [1] [1]. At low temperature, the molecules move slowly so intermolecular attractions are no longer negligible [1] [1]. These are precisely the two assumptions of the model that fail.

4. (a) Diamond — giant covalent [1]. Graphite — giant covalent (layered) [1]. Iodine — simple molecular [1].

(b) Each carbon in graphite forms only three covalent bonds [1], leaving one delocalised electron per atom [1] which is free to move along the layers, carrying charge. In diamond all four outer electrons are used in covalent bonds, so none are free [1].

(c) Only the weak instantaneous dipole–induced dipole forces between I₂ molecules must be overcome to melt iodine, giving it a low melting point of 114 °C [1]; because these intermolecular forces are weak, solid iodine also has a significant vapour pressure below 114 °C, so it visibly sublimes at temperatures below its actual melting point too [1].

5. In the solid the ions are held in fixed positions and cannot move [1]; when molten they are free to move and carry charge [1].

6. Copper has a giant metallic structure, held together by electrostatic attraction between metal cations and a “sea” of delocalised electrons [1]. This delocalised electron sea is mobile in both the solid and molten state [1], so copper can carry charge either way — unlike an ionic solid, whose ions are only free to move once molten [1].

7. A high melting point rules out simple molecular structure, since only weak intermolecular forces would need overcoming there [1]. Conducting only when molten, not as a solid, rules out giant metallic, which conducts in both states [1], and also rules out giant molecular, which (except graphite) does not conduct at all [1]. Conducting when molten, together with solubility in water, is the signature of mobile ions being released into solution [1] — the solid is giant ionic [1].

8. In graphite each carbon atom is bonded to three others, so one electron per atom is delocalised [1]; these delocalised electrons are free to move along the layers and carry charge [1]. In diamond each carbon atom uses all four outer electrons in covalent bonds, so there are no delocalised (mobile) electrons [1].

Examiner insight (June 2025): many answers said “graphite has delocalised electrons” without saying the electrons move, which is what conduction needs; some wrote only “giant structure”. Try the real question next: Cambridge International AS & A Level Chemistry 9701, June 2025, Paper 22, Question 1.

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Where marks are usually lost

• Using cm³ or °C in the ideal gas equation.
• Saying covalent bonds break when iodine sublimes, or calling 114 °C a sublimation temperature rather than iodine’s melting point.
• Saying graphite’s fourth electron is “unbonded” rather than delocalised.
• Explaining real-gas deviation without naming the specific failing assumptions.
• Confusing “conducts when molten” (ionic) with “conducts in both states” (metallic) — the deduction hinges on exactly this distinction.
• Treating simple molecular structures as weakly bonded overall, when it is only the intermolecular forces that are weak — the covalent bonds within each molecule are just as strong as in a giant covalent structure.
• Forgetting that ice is held together by hydrogen bonding — the strongest type of van der Waals’ force — rather than by id-id (London dispersion) forces alone, which is why it has an unusually open structure and floats on liquid water.

The four structure types at a glance

Structure	Examples	Conducts as solid?	Conducts when molten?
Giant ionic	NaCl, MgO	No	Yes
Simple molecular	iodine, ice	No	No
Giant molecular	diamond, SiO₂ (graphite excepted)	No	No
Giant metallic	copper	Yes	Yes

Working through this table by elimination — as in Question 7 — is the standard technique for a “deduce the structure” question.

For condensed recall notes on this topic, see the Ideal Gases and Structure revision notes; for the full explanation with additional worked examples, see the Ideal Gases and Structure study guide.