To calculate the enthalpy change for the complete combustion of 2 moles of propane C3H8 in the presence of excess oxygen gas O2 , we first need to write the balanced chemical equation for the combustion reaction:C3H8 + 5O2 3CO2 + 4H2ONow, we need to find the standard enthalpy of formation Hf for each compound involved in the reaction. The values are as follows in kJ/mol :Hf C3H8 = -103.85Hf O2 = 0 since O2 is in its standard state Hf CO2 = -393.5Hf H2O = -285.8Next, we can use Hess's Law to calculate the enthalpy change for the reaction:H = [3 * Hf CO2 + 4 * Hf H2O ] - [2 * Hf C3H8 ]H = [3 * -393.5 + 4 * -285.8 ] - [2 * -103.85 ]H = -1180.5 + -1143.2 - -207.7 H = -2323.7 + 207.7H = -2116 kJ/molSince we are considering the combustion of 2 moles of propane, we need to multiply the enthalpy change by 2:H 2 moles of C3H8 = -2116 * 2 = -4232 kJSo, the enthalpy change for the complete combustion of 2 moles of propane C3H8 in the presence of excess oxygen gas O2 is -4232 kJ.