Why is Enthalpy of Compound at Some Temperature Equal to its Enthalpy of Formation at the Same Temperature? This is a question I don't understand in thermo. Enthalpy of compound would be sum of U + PV at some temperature, but since we can't calculate absolute value of H, we calculate it relatively to some reference. Enthalpy of formation is heat needed to be exchanged between system and surroundings at some pressure to keep the system at the same temperature when compound is formed from elements in the most stable state. Why are these two quantities equal?
They are not equal.  The enthalpy of a compound at temperature T is relative to the pure elements at the reference temperature $T^0$.  So it is equal to the standard heat of formation at the reference temperature plus $C_p(T-T^0)$.