${\text{N}}_{\text{2}\left(\text{g}\right)}{\text{+3H}}_{\text{2}\left(\text{g}\right)}\stackrel{}{\to }{\text{2NH}}_{\text{3}\left(\text{g}\right)}\text{ΔH=-46kcal}$.  Then heat of formation of ${\text{NH}}_{\text{3}}$is

Heat of formation is calculated per mole.

$$\begin{gathered} {\mathrm{N}}_2+3{\mathrm{H}}_2\to 2{\mathrm{NH}}_3; ∆\mathrm{H}=-46 Kcal \\ For formation of 2 mole of N{H}_3= -46kcal \\ For for mation of 1 mol of N{H}_3= -\frac{46}{2}=-23kcal \end{gathered}$$