Given 3 thermochemical eq.,calculate the standard enthalpy of formation (in kilojoules per mole) of CuO(s).

Given the following thermochemical equations, calculate the standard enthalpy of formation (in kilojoules per mole) of CuO(s).

can you help me with this? i'm confused on how to do this :(

2Cu(s) + S(s) Cu2S(s) [ΔrH° = –79.5 kJ mol-1]
S(s) + O2(g) SO2(g) [ΔrH° = –297 kJ mol -1]
Cu2S(s) + 2O2(g) 2CuO(s) + SO2(g) [ΔrH° = –527.5 kJ mol-1]

Here we apply the Hess's Law of constant heat summation.
We also know that ;
Cu +(1/2) O2 =CuO

We have to invert the 2nd equation and hence we get Hr=+297.0 kJ mol-1.

Noe adding the above three equations we get ;
2Cu + O2 = 2CuO (All the rest of the terms on either side of the equations cancel.The addition follows simple algebraic laws.)

Hence net enthalpy of formation of 2CuO = -79.5+297-527.5= -310.0 kJ mol-1.

Now the enthalpy of formation of CuO IS -310/2=-155 kJ mol-1.

For better under standing of Hess's Law go here http://en.wikipedia.org/wiki/Hess's_law.

Hope this helped. :)