Given : C2H2(g)+H2(g)⟶C2H4(g):ΔH∘=−175kJmol−1Enthalpy  of  C2H4 = 50 kJ/mole Heat of combustion of C and H2 are respectively 390 and 280 kJ/mole If ΔH∘ is enthalpy of combustion (in kJmol−1 ) of C2H2(g), then calculate the value of ΔH∘257.