This is the balanced eq N2 + 3H2 -> 2NH3 first you need to find mole of N2 by using mol = mass ÷ molar mass. mol N2= 20g ÷ (14.01×2)g/mol =0.7138mol then look at the coefficient between H2 and NH3. it is N2:NH3 1:2 0.7138:0.7138×2 0.7138:1.4276 moles moles of NH3 = 1.4276 moles