To calculate the standard enthalpy of formation of methane gas CH4 , we can use the following combustion reaction:CH4 g + 2 O2 g CO2 g + 2 H2O l The standard enthalpy change for this reaction, Hcombustion, is given as -890.36 kJ/mol. We also have the standard enthalpies of formation for H2O l and CO2 g . We can use Hess's Law to determine the standard enthalpy of formation for CH4 g .Hess's Law states that the total enthalpy change for a reaction is the sum of the enthalpy changes for each step in the reaction, regardless of the order of the steps. In this case, we can write the following equations:1. CH4 g C graphite + 2 H2 g ; Hf = x kJ/mol unknown 2. C graphite + O2 g CO2 g ; Hf = -393.5 kJ/mol3. 2 H2 g + O2 g 2 H2O l ; Hf = 2 * -285.8 kJ/mol = -571.6 kJ/molNow, we can add equations 2 and 3 to get the combustion reaction:C graphite + 2 H2 g + 2 O2 g CO2 g + 2 H2O l The enthalpy change for this reaction is the sum of the enthalpy changes for equations 2 and 3:Hcombustion = -393.5 kJ/mol + -571.6 kJ/mol = -965.1 kJ/molHowever, we are given that the actual Hcombustion of methane is -890.36 kJ/mol. Therefore, the difference between these two values is the standard enthalpy of formation of CH4 g :x = -890.36 kJ/mol - -965.1 kJ/mol = 74.74 kJ/molSo, the standard enthalpy of formation of methane gas CH4 is 74.74 kJ/mol.