The enthalpy of formation of water H2O can be calculated using the following balanced chemical equation:H2 g + 1/2 O2 g H2O l The enthalpy of formation Hf of a compound is the change in enthalpy when 1 mole of the compound is formed from its constituent elements in their standard states. In this case, we are given the enthalpies of formation for hydrogen gas H2 and oxygen gas O2 , which are -286 kJ/mol and 0 kJ/mol, respectively.Since the enthalpy of formation of the elements in their standard states is zero, we can use the following equation to calculate the enthalpy of formation of water:Hf H2O = Hf H2 + 1/2 Hf O2 Hf H2O = -286 kJ/mol + 1/2 0 kJ/mol Hf H2O = -286 kJ/molTherefore, the enthalpy of formation of water H2O is -286 kJ/mol.