Question: how to calculate standard enthalpy of formation of water?
Answer: 2H2 + O2 -> 2H2O. This means that for every 2 molecules of Hydrogen gas, one molecule of Oxygen is needed to make 2 molecules of water. ... 
Enthalpy of reaction = (Enthalpy of products) - (Enthalpy of reactants) ... 
Enthalpy= -285.83-(0+0)
This means Enthalpy of formation of water=-285.83kJ/mol.