Chemistry
posted by Luc .
∆H⁰rxn, for the reaction of Ca(s) + 2H+(aq) > Ca2+(aq) + H2(g) is 544 kJ/mol.
∆H⁰rxn, for the reaction of CaO(s) + 2H+(aq) > Ca2+(aq) + H2O(l) is 193 kJ/mol.
Based on these results, what is the ∆Hf for CaO(s) [in kJ/mol] if the heat of formation of water is 286 kJ/mol?
Can someone please help me with this. I calculated all the given information above; however, I have no idea how to connect the dots and answer this question. Thank you!

Add equation 1 to the reverse of equation 2 to equation 3. Then add the delta Hs; remember to reverse the sign if you reverse the reaction. The final reaction will be
Ca(s) + 1/2 O2 > CaO(s) Notice that everything else cancels.
Delta H for the desired reaction will be 544 + 193  286 = ?? kJ/mol. 
637 kJ/mol.
okay, I understand now... thanks Dr. Bob! 
thanks Dr. bob.

I don't understand why you add the water heat of formation. Can someone please explain this, it doesn't make any sense to me.