The heat of combustion is determined as the heat is released. The vessel and its contents are then cooled to the original 25 ☌.
Hydrogen and oxygen react during combustion, producing water vapor. The reaction is then allowed to complete. A stoichiometric mixture of fuel and oxidizer is kept in a steel container at 25 ☌ and initiated by an ignition device. The heat of combustion is calculated by using a bomb calorimeter. When the amount is quantified for one mole of the substance, we call it molar heat of combustion. The heat of combustion is the heat released by a substance when it undergoes complete combustion in the presence of oxygen at standard temperature and pressure conditions.
