To calculate the standard enthalpy of formation of water, we need to consider the balanced chemical equation for the formation of water from its elements in their standard states:H2 g + 1/2 O2 g H2O l The standard enthalpy of formation of an element in its standard state is zero. Therefore, the standard enthalpy of formation of H2 g and O2 g are both 0 kJ/mol.Now, let's consider the combustion reactions of hydrogen and oxygen:1. H2 g + 1/2 O2 g H2O l ; H = -286 kJ/mol given 2. O2 g 1/2 O2 g ; H = 0 kJ/mol since it's an element in its standard state Now, we can use Hess's Law to calculate the standard enthalpy of formation of water. Hess's Law states that the enthalpy change of a reaction is the same, whether it occurs in one step or several steps. In this case, we can add the enthalpy changes of the two reactions above to find the enthalpy change for the formation of water:Hf H2O = H1 + H2Hf H2O = -286 kJ/mol + 0 kJ/molHf H2O = -286 kJ/molTherefore, the standard enthalpy of formation of water is -286 kJ/mol.