Dinitrogen pentoxide is formed by the sequence

I $N_2(g) + O_2(g) \rightarrow 2NO(g)$ $\Delta H = +180\ kJ\,mol^{-1}$
II $NO(g) + \frac{1}{2}O_2(g) \rightarrow NO_2(g)$ $\Delta H = -57\ kJ\,mol^{-1}$
III $2NO_2(g) + \frac{1}{2}O_2(g) \rightarrow N_2O_5(g)$ $\Delta H = -55\ kJ\,mol^{-1}$

The enthalpy change of formation of $N_2O_5(g)$, in $kJ\,mol^{-1}$, is