How do I find the molar enthalpy H (kJ/mol) for reaction of NH3 + HCl -> NH4Cl at temp 9.8 celsius by using Hess's Law. I do not understand why there are unknown variables and how to use that to find the molar enthalpy. PLEASE HELP.
1/2 N2 + 3/2 H2 + xH2o -> NH3 H= -80.7 kJ
1/2 H2 + 1/2 Cl2 + yH2o -> HCl H= -164.7 kJ