Henry thinks that a mole contains  $6.023\times {10}^{24}$ molecules.  Hence the mass of Henry’s mole of Nitrogen is

1 molecule      ____     $28\times 1.66\times {10}^{-24}gms$

 $6.023\times {10}^{24}$ molecules   ____  mol. wt  ( according to Henry)

 $\therefore mol.wt=6.023\times {10}^{24}\times 28\times 1.66\times {10}^{-24}=280 gms$