Using Hess’s law, what is the standard enthalpy of formation of manganese (II) oxide, MnO(s)?

see explanation

Explanation:

By definition, the Standard Heat of Formation (ΔH°(f)) is the heat gained or lost on formation of a molecular substance from its basic elements in their standard state (25°C, 1atm).

For MnO(s) and table of Standard Thermodynamic Properties, the standard state for manganese is Mn°(s) and oxygen O₂(g) which combine to form standard state manganese(II) oxide (MnO(s)) as follows:

Mn°(s) + 1/2O₂(g) => MnO(s); ΔH°(f) = -384.9 Kj/mole