Stoichiometry and Moles

STOICHIOMETRY is the math behind chemistry. It lets you predict EXACTLY how much of each chemical you need for a reaction and how much product you'll get. Every pharmaceutical, every industrial chemical, every rocket fuel calculation uses stoichiometry. Today: the core concepts.

A MOLE = 6.022 × 10²³ particles. This crazy number (Avogadro's Number) is chosen so that the atomic mass of an element (in amu) equals 1 mole's weight in grams.\n\n- 1 mole of Hydrogen = 1 gram\n- 1 mole of Carbon = 12 grams\n- 1 mole of Oxygen = 16 grams\n- 1 mole of Sodium = 23 grams\n\nSo if you weigh out 12 grams of carbon, you have 6.022 × 10²³ carbon atoms. That is 1 mole.

The MOLAR MASS of a compound = sum of atomic masses.\n\nExample: H₂O (water)\n- 2 H × 1 = 2\n- 1 O × 16 = 16\n- **Total molar mass = 18 g/mol**\n\nNaCl (salt)\n- 1 Na = 23\n- 1 Cl = 35.5\n- **Total = 58.5 g/mol**\n\nUse the periodic table to look up atomic masses.

**Grams → Moles:** divide by molar mass.\n\nHow many moles in 36 g of water?\n36 ÷ 18 = 2 moles.\n\n**Moles → Grams:** multiply by molar mass.\n\nHow many grams of NaCl in 3 moles?\n3 × 58.5 = 175.5 grams.\n\nThese two conversions are used constantly.

Chemical reactions must BALANCE — same number of each atom on both sides (Conservation of Mass).\n\nUnbalanced: H₂ + O₂ → H₂O\n\nLeft: 2 H, 2 O\nRight: 2 H, 1 O\n\nO is unbalanced. Multiply H₂O by 2: H₂ + O₂ → 2 H₂O\n\nNow: 2 H vs 4 H. Multiply H₂ by 2: 2 H₂ + O₂ → 2 H₂O\n\nCheck: 4 H, 2 O on both sides. BALANCED!

Given a balanced equation, predict amounts of products.\n\nN₂ + 3 H₂ → 2 NH₃\n\nIf you have 10 moles H₂, how many moles NH₃?\n\nRatio (from equation): 3 H₂ : 2 NH₃\n\n10 moles H₂ × (2 NH₃ / 3 H₂) = 6.67 moles NH₃\n\nThis is how chemical engineers scale up reactions from lab to factory.

Sometimes one reactant runs out first — that's the LIMITING REACTANT.\n\nTo find it:\n1. Calculate moles of each reactant available\n2. Calculate moles of product each COULD make using ratios\n3. The reactant making the LEAST product = limiting\n\nThis matters in industry: running out of one ingredient stops the whole reaction.