How many kilojoules of heat are absorbed when 40.7g of MgO(s) is decomposed into Mg(s) and O2 (g) at constant pressure?

Answer 1

2Mg + O2 ==> 2MgO
Look up dHf for MgO, which will be some number/mol and multiply by 2 for 2MgO.
See this link to find it.
http://en.wikipedia.org/wiki/Standard_enthalpy_change_of_formation_%28data_table%29
Then dHf x [(40.7/(2*40.3)] = ? and change the - sign to +.

Answer 2

0.5

Answer 3

-30.1 kj