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If heat of reaction, Ca(s) + 2H+(aq) Ca2+(aq) + H2(g) , is - 544 kJ/mol, heat of reaction, CaO(s) + 2H+(aq) Ca2+(aq) + H2O(l) , is -192 kJ/mol. and heat of formation of water is -286, what is heat of CaO?

Ca(s) etc is equation 1.
CaO + 2H^+ etc is equation 2.
H2 + 1/2 O2 ==> H2O is equation 3.

Write equation 1 as is.
Reverse equation 2 and change sign of delta H.
Write equation 3 as is.

Add all the equations (1, reversed 2, and 3) and see if that won't give you the formation of CaO; i.e., Ca(s) + 1/2 O2 ==> CaO.
Then add delta H1 + reversed delta H2 with sign changed + delta H 3 for H2O as is. The total should be the delta H for the end equation.

Post your work if you get stuck.

i found that the formation of CaO is -638. However, I'm confused about the next step to find delta H for the end equation.

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