To calculate the standard enthalpy of formation of methane CH4 , we need to use the standard enthalpies of formation for CO2 and H2O, as well as the balanced chemical equation for the combustion of methane:CH4 g + 2 O2 g CO2 g + 2 H2O l The standard enthalpy change for this reaction H can be calculated using Hess's law, which states that the enthalpy change for a reaction is the sum of the standard enthalpies of formation of the products minus the sum of the standard enthalpies of formation of the reactants:H = [Hf CO2 + 2Hf H2O ] - [Hf CH4 + 2Hf O2 ]Since O2 is an element in its standard state, its standard enthalpy of formation is zero. Therefore, the equation becomes:H = [Hf CO2 + 2Hf H2O ] - Hf CH4 We are given the standard enthalpies of formation for CO2 and H2O:Hf CO2 = -393.5 kJ/molHf H2O = -285.8 kJ/molThe standard enthalpy change for the combustion of methane H is also known:H = -890.4 kJ/mol This value can be found in standard chemistry reference materials. Now we can solve for the standard enthalpy of formation of methane Hf CH4 :-890.4 kJ/mol = [ -393.5 kJ/mol + 2 -285.8 kJ/mol ] - Hf CH4 -890.4 kJ/mol = -965.1 kJ/mol - Hf CH4 Hf CH4 = -965.1 kJ/mol + 890.4 kJ/molHf CH4 = -74.7 kJ/molTherefore, the standard enthalpy of formation of methane CH4 is -74.7 kJ/mol.