#### Answer

602 kJ

#### Work Step by Step

The question asks about the decomposition of MgO, which is the reverse of the reaction shown. Therefore, if we reverse the reaction, we must also reverse the sigh on $\Delta H$:
2MgO --> 2Mg + O2 $\Delta H = +1204 kJ$
Now, to find how much heat is absorbed when 40.3g are decomposed, we must first convert this to moles. We need the molar mass of MgO.
Mg has a mass of 24.30
O has a mass of 15.994
MgO has a mass of 40.29 g/mol
Therefore, 40.3 g is:
40.3g * (1 mol/40.3g) = 1 mole and since the enthalpy released is 1204 kJ for 2 moles of MgO, then we divide this number by 2 and get
602 kJ for one mole of MgO