Chemistry / Unit 4 Chem 0697 worked example, 60 to 90 seconds
Calculating Heat Released by Combustion of a Given Mass
The idea
Calculating the heat released by burning a given mass of fuel means converting that mass to moles using the fuel's molar mass, then multiplying by its standard enthalpy of combustion, which is defined on a per-mole basis rather than per gram.
In practice
One liter of isooctane, at a density of 0.692 g/mL, has a mass near 692 g. Dividing 692 g by isooctane's molar mass, about 114.23 g/mol, gives roughly 6.06 mol of isooctane in that liter. Multiplying 6.06 mol by isooctane's standard enthalpy of combustion, about −5460 kJ per mole, gives approximately −3.3 × 10⁴ kJ released by burning the full liter, an amount large enough to melt roughly 99 kg of ice.
Where people slip
Multiplying the fuel's mass in grams directly by its per-mole enthalpy of combustion, skipping the conversion to moles entirely, produces an answer wrong by the fuel's molar mass.
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