What is the standard molar enthalpy change of formation of O2 (g) at 25 °C?

Zero

Explanation:

Standard Heat of Formation (ΔH°f) for any element in it's basic standard state is zero (0) Kj/mole. If you have a college general chemistry text, look in the appendix for table with title 'Thermodynamic Properties of Substances at 25°C'. Scan down under the symbol (ΔH°f) ... Note that the substance listed by the 'zero' Kj/mole values will be an element. This is the element in basic standard state. Oxygen (O₂ (g)) will have a 0 Kj/mole value as does H₂ (g), N₂ (g), F₂ (g), Cl₂ (g), Br₂ (l) and I₂ (s). These are the 7 diatomic elements in basic standard state. Look at the pattern that is made for this set of elements on the periodic table. Starting with N > O > F > Cl > Br > I it traces as a '7' = > You just have to remember Hydrogen is a member of the set also.