As we know that the calorific value of a fuel is same as the heat of combustion.Given that the calorific value of H2 gas is xkJ/gm
Molar mass of H2 gas =2g
Therefore, calorific value of H2 gas per mole =x2kJ/mol
Hence the the heat of combustion of H2 is x2kJ/mol.
Combustion of H2-
H2+12O]2⟶H[2O
From the above reaction, the heat of formation of H2O is same as the heat of combustion of H2 gas.
Hence the heat of formation of H2O is x2kJ/mol.