To calculate the enthalpy change of combustion for the reaction of methane when 1.25 moles of methane is burnt, we can use the given enthalpy change of combustion for methane and multiply it by the number of moles.Enthalpy change of combustion for methane = -890.3 kJ/molNumber of moles of methane = 1.25 molesEnthalpy change for 1.25 moles of methane = enthalpy change of combustion for methane number of moles Enthalpy change for 1.25 moles of methane = -890.3 kJ/mol 1.25 moles Enthalpy change for 1.25 moles of methane = -1112.875 kJSo, the enthalpy change of combustion for the reaction of methane when 1.25 moles of methane is completely burnt in excess oxygen gas at standard conditions is -1112.875 kJ.