To calculate the standard enthalpy change for the formation of methane gas CH4 from its elements, we can use the following equation:Hf CH4 = Hf products - Hf reactants The balanced chemical equation for the formation of methane is:C graphite + 2H2 g CH4 g Now, we can plug in the given values:Hf CH4 = [Hf CH4 ] - [Hf C graphite + 2 * Hf H2 g ]Since Hf C graphite and Hf H2 g are both 0 kJ/mol, the equation simplifies to:Hf CH4 = [Hf CH4 ] - [0 + 2 * 0]We are given the value of Hc CH4 which is -890.4 kJ/mol. The enthalpy of combustion Hc is the enthalpy change when one mole of a substance is completely burned in oxygen. In this case, we are given the enthalpy of combustion for methane, which is the reverse reaction of the formation of methane.The enthalpy change for the reverse reaction is the negative of the enthalpy change for the forward reaction. Therefore, the enthalpy change for the formation of methane Hf CH4 is the negative of the enthalpy of combustion Hc CH4 :Hf CH4 = - -890.4 kJ/mol = 890.4 kJ/molSo, the standard enthalpy change for the formation of methane gas CH4 from its elements is 890.4 kJ/mol.