Consider the balanced reaction,
N₂ + 3 H₂ ⇒ 2 NH₃
which says that 1 mole of nitrogen gas combines with 3 moles of hydrogen gas to produce 2 moles of ammonia gas.
Look up the molar masses of each element:
• N has a molar mass of 14.007 g/mol
• H has a molar mass of 1.008 g/mol
Then the reaction starts with
• (35.6 g) (1/28.014 mol/g) ≈ 1.27 mol N₂
• (7.69 g) (1/2.016 mol/g) ≈ 3.81 mol H₂
which means we have nearly exactly 3 times as many moles of H₂ as we do of N₂. These will combine to make 2 * 1.27 = 2.54 moles of NH₃, which has a molar mass of 17.031 g/mol, and so the reaction produces
(2.54 mol) (17.031 g/mol) ≈ 43.3 g NH₃
