The coefficients of a balanced equation are ratios of moles, not masses: in N₂ + 3 H₂ → 2 NH₃, 28.0 g of nitrogen needs 6.05 g of hydrogen, not 84 g, and makes 34.0 g of ammonia.
A reaction rearranges atoms, so the equation counts molecules; the mass of a mole of each is its molar mass. 1 mol of nitrogen is 28.014 g and 3 mol of hydrogen is 3 × 2.016 = 6.048 g, together 34.062 g, which is the mass of the 2 mol of ammonia they make. Convert grams to moles, use the coefficients, convert back.
It assumes the reaction goes to completion as written. Ammonia synthesis in practice stops at an equilibrium mixture, and a real reaction loses some in side reactions; that gap is the percentage yield. The equation says what can be made, not what is.
Estimate with the rule, then check it against the calculator that models it properly.
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