To calculate the standard enthalpy of formation of water H2O , we can use the following reaction:H2 g + 1/2 O2 g H2O l First, we need to find a reaction that involves H2O and the given information. We can use the decomposition of hydrogen peroxide H2O2 as follows:2 H2O2 l 2 H2O l + O2 g H = +188.2 kJ/mol since the given Hf of H2O2 l is -188.2 kJ/mol, the reverse reaction will have the opposite sign Now, we can manipulate this reaction to match the desired reaction for the formation of water:1/2 2 H2O2 l 2 H2O l + O2 g H = 1/2 +188.2 kJ/mol = +94.1 kJ/molThis reaction is now:H2O2 l H2O l + 1/2 O2 g H = +94.1 kJ/molNext, we need to find a reaction that involves H2O2 and the given information. We can use the formation of hydrogen peroxide as follows:H2 g + O2 g H2O2 l H = -188.2 kJ/molNow, we can add the two reactions to get the desired reaction for the formation of water:H2 g + O2 g H2O2 l H = -188.2 kJ/molH2O2 l H2O l + 1/2 O2 g H = +94.1 kJ/mol----------------------------------------------H2 g + 1/2 O2 g H2O l H = -94.1 kJ/molTherefore, the standard enthalpy of formation of water H2O is -94.1 kJ/mol.