To calculate the standard enthalpy change for the complete combustion of methane gas, we first need to write the balanced chemical equation for the combustion reaction:CH g + 2O g CO g + 2HO l Next, we need to know the standard enthalpy of formation for all the compounds involved in the reaction. We are given the values for CH and HO, but we also need the value for CO. The standard enthalpy of formation for CO is -393.5 kJ/mol.Now, we can use Hess's Law to calculate the standard enthalpy change for the reaction:H = [Hf products ] - [Hf reactants ]H = [1 mol CO -393.5 kJ/mol + 2 mol HO -285.8 kJ/mol ] - [1 mol CH -74.9 kJ/mol + 2 mol O 0 kJ/mol ]H = -393.5 kJ + 2 -285.8 kJ - -74.9 kJ H = -393.5 kJ - 571.6 kJ + 74.9 kJH = -965.1 kJ + 74.9 kJH = -890.2 kJThe standard enthalpy change for the complete combustion of methane gas at 25C and 1 atm pressure is -890.2 kJ/mol.