Hydrogen-Oxygen fuel cell is with an energy efficiency of 70%.  The ΔHFormation0H2O(l)=−286.5kJ/mole.  The amount of electrical energy generated by the fuel cell with the consumption of 4g of Hydrogen in kJ is

401.1 kJ of electrical energy Number of mole of Hydrogen consumed = 4g2g mole−1=2moleNumber of mole of water formed = 2 moleAmount of heat energy that is released during the formation of 2 mole of water= 2 x 286.5 = 573 KJ70% of the above energy is converted into electrical energy= 573 x 0.7 = 401.1 KJ