The molar heat of reaction, H rxn , for the reaction of

Question

Ca(s) + 2H+(aq)-> Ca2+(aq) + H2(g)

was found to be -550kJ/mol

The change Hrxn , for the reaction of

CaO(s) + 2H+(aq)-> Ca2+(aq) + H2O(l)

was found to be -189kJ/mol

Based on these results if the formation of water is -280kJ/mol What is the change in heat formation for CaO (s) ______ kJ/mol

I was thinking of doing Hess's Law but I am not really sure where to go about this problem.

Answer 1

Ca(s) + 2H+(aq)-> Ca2+(aq) + H2(g)
CaO(s) + 2H+(aq)-> Ca2+(aq) + H2O(l)
H2 + 1/2 O2 ==> H2O

Leave equation 1 as is.
Reverse equation 2.
Leave equation 3 as is.
Add the three equations to obtain
Ca + O2 ==> CaO which is what you want is I didn't goof. Post your work if you get stuck. BE SURE and check my work.
(Note: Since I multiplied all three equations by 2 and had to divide the added equations by 2; I expect just omitting all of the multiplications will work just fine.