AS Chemistry: Ideal Gases and Structure — Practice Questions

These are original questions written for Marlbridge, in the style and at the standard of the examination. They are not reproduced past-paper questions — 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) Explain why iodine sublimes at only 114 °C. [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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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 induced dipole–induced dipole forces between I₂ molecules must be overcome [1]; the strong covalent bonds within each molecule are not broken [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].

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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.
• 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 partly by hydrogen bonding rather than only van der Waals forces, 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.