To calculate the heat of formation of a water molecule from its elements, we need to consider the balanced chemical equation for the formation of water from hydrogen and oxygen gas:2 H2 g + O2 g 2 H2O l The standard enthalpy of formation Hf of a compound is the change in enthalpy when 1 mole of the compound is formed from its elements in their standard states. In this case, we are given the standard enthalpy of formation of hydrogen gas H2 as -285.8 kJ/mol and oxygen gas O2 as 0 kJ/mol. However, these values are not directly useful for our calculation, as they refer to the formation of the elements themselves, not the formation of water.Instead, we need the standard enthalpy of formation of water H2O . This value is a known constant and is equal to -285.8 kJ/mol for liquid water. Now, we can use this value to calculate the heat of formation of a water molecule.The balanced chemical equation tells us that 2 moles of hydrogen gas react with 1 mole of oxygen gas to form 2 moles of water. Therefore, the heat of formation for 1 mole of water can be calculated as follows:Hf H2O = -285.8 kJ/molSince we are interested in the heat of formation for a single water molecule, we can divide this value by Avogadro's number 6.022 x 10^23 molecules/mol to obtain the heat of formation per molecule:Hf H2O, per molecule = -285.8 kJ/mol / 6.022 x 10^23 molecules/mol = -4.74 x 10^-22 kJ/moleculeThe negative sign of the enthalpy of formation indicates that the formation of a water molecule from its elements is an exothermic process, meaning that energy is released during the reaction. This is consistent with the fact that the formation of water from hydrogen and oxygen gas is a highly exothermic reaction. The negative enthalpy of formation also suggests that the water molecule is more stable than its constituent elements, as energy is released when the elements combine to form the more stable compound.