the enthalpy of NH3 (g) is 46.2kjmole.the heat of the reaction 2NH3(g) gives rise to N2+3H2(g)
N2(g) + 3H2(g) ------> 2NH3(g) ; rH° = -92.4 KJ/mol
enthalpy of formation of NH3 means heat released in the formation of 1 mole of NH3 .
so, divide both sides by 2 in above reaction
1/2N2(g) + 3/2H2(g) ---> NH3(g) ;
Therefore , fH° = rH°/2 = -92.4/2 = 46.2 KJ/mol
Thank you.