Lesson 6: Stoichiometry & Chemical Reactions
Stoichiometry is the math of chemistry — it tells you exactly how much of each reactant you need and how much product you'll get. Every chemical equation is a precise recipe, and the coefficients are the serving sizes.
Key Concepts
Law of Conservation of Mass
Atoms are never created or destroyed in a chemical reaction — only rearranged. A balanced equation has the same number of each type of atom on both sides.
Balancing Equations
Add whole-number coefficients in front of formulas to make both sides equal. Never change subscripts (that would change the substance). Example: H₂ + O₂ → H₂O becomes 2H₂ + O₂ → 2H₂O.
The Mole
A mole is 6.022 × 10²³ particles (Avogadro's number). One mole of any substance has a mass equal to its molar mass in grams. Moles let us count atoms by weighing.
Mole Ratios
Coefficients in a balanced equation give mole ratios. In 2H₂ + O₂ → 2H₂O, the ratio is 2:1:2. If you use 4 mol H₂, you need 2 mol O₂ and produce 4 mol H₂O.
Limiting Reagent
The reactant that runs out first limits how much product forms. The other reactant is in excess. Stoichiometry lets you identify and calculate around the limiting reagent.
🔬 Virtual Lab: Reaction Coefficient Balancer
Adjust the coefficients with + / − buttons. The atom counter updates live. Green = balanced, Red = not balanced yet.
✅ Check Your Understanding
1. What does balancing a chemical equation ensure?
2. In 2H₂ + O₂ → 2H₂O, what is the mole ratio of H₂ to O₂?
3. The reactant that is completely consumed first in a reaction is the: