Simple molecular structures
4 marks · NormalThe table shows the relative molecular mass and boiling points for two gaseous substances:
| Substance | Relative molecular mass (Mr) | Boiling point (°C) |
|---|---|---|
| Methane | 16 | –161 |
| Sulfur hexafluoride | 146 | –64 |
Explain why both methane and sulfur hexafluoride are gases at room temperature despite having different molecular masses.
Explain how increasing the molecular mass affects the boiling point of simple molecular substances, using these examples.
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(a) — · must show both have simple molecular structures, weak intermolecular forces allow them to be gases at room temperature
(b) — · must show higher molecular mass means stronger intermolecular forces, higher boiling point for substance with greater Mr
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Marking points
B1 (a) · 1 mark · Both methane and sulfur hexafluoride have simple molecular structures/gas at room temperature due to this structure.
B2 (a) · 1 mark · Intermolecular forces are weak in both, so they remain as gases at room temperature.
B3 (b) · 1 mark · Boiling point increases as molecular mass increases due to stronger intermolecular forces.
B4 (b) · 1 mark · Sulfur hexafluoride (higher Mr) has a higher boiling point than methane (lower Mr).
Worked solution
a) Both methane and sulfur hexafluoride have simple molecular structures with weak intermolecular forces between molecules. These weak forces mean both substances are gases at room temperature.
b) As molecular mass increases, intermolecular forces become stronger, so the boiling point rises. Sulfur hexafluoride (Mr = 146) boils at a much higher temperature (–64°C) than methane (Mr = 16, boiling point –161°C).