N2+3H2-->2NH3
first we find the number of nitrogen moles
the molar mass for N2 is 14+14=28g/mole
to find the number of moles we divide the mass by the molar mass
n=400/28=14.28 moles
for 1 mole of N2 we use 3 moles of H2, therefore for 14.28 moles of N2 we use 14.28*3=42.84 moles ofH2
to find the mass of H2 we multiply the number of moles by the molar mass
the molar mass of H2 is 2g/mole
2*42.84=85.68g of H2