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When 5 litres of a gas mixture of methane and propane is perfectly combusted at 0°C and 1 atmosphere, 16 litre of oxygen at the same temperature and pressure is consumed. The amount of heat released from this combustion in kJ (ΔHcomb (CH4) = 890 kJ mol⁻¹, ΔHcomb (C3H8) = 2220 kJ mol⁻¹) is:

1. 32
2. 38
3. 317
4. 47

Correct answer: 317

Solution

The combustion reactions are CH4 + 2O2 → CO2 + 2H2O and C3H8 + 5O2 → 3CO2 + 4H2O. Let the volumes of CH4 and C3H8 be x and y liters, respectively. From stoichiometry, x + y = 5 and 2x + 5y = 16. Solving these gives x = 2 L and y = 3 L. Heat released is (2/22.4) × 890 + (3/22.4) × 2220 = 317 kJ.

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