Overview

Stoichiometry is the math of chemistry. It allows chemists to predict how much product will form from a given amount of reactant, or how much reactant is needed to make a specific amount of product.

Core Idea

Mole Ratio: The coefficients in a balanced chemical equation tell you the ratio of moles of each substance. $$ 2H_2 + O_2 \rightarrow 2H_2O $$ (2 moles of Hydrogen + 1 mole of Oxygen $\rightarrow$ 2 moles of Water)

Formal Definition (if applicable)

Mole: The SI unit for amount of substance ($6.022 \times 10^{23}$ particles). Limiting Reactant: The reactant that is completely consumed first, limiting the amount of product formed.

Intuition

It’s like a recipe. If a sandwich needs 2 slices of bread and 1 slice of cheese, and you have 10 slices of bread but only 3 slices of cheese, you can only make 3 sandwiches. The cheese is the limiting reactant.

Examples

  • Baking: Calculating how much flour you need for 50 cookies.
  • Industrial Chemistry: Calculating how much raw material is needed to produce a ton of fertilizer.
  • Airbags: Calculating the exact amount of chemical needed to inflate the bag instantly without exploding.

Common Misconceptions

  • “1 gram of A reacts with 1 gram of B.” (No! You must convert to moles first because atoms have different masses.)
  • “The reaction stops when all reactants are gone.” (It stops when the limiting reactant is gone.)
  • Molar Mass: The mass of one mole of a substance.
  • Percent Yield: (Actual Yield / Theoretical Yield) $\times$ 100%.
  • Concentration (Molarity): Moles per liter.

Applications

  • Pharmacology: Dosing medications correctly.
  • Environmental Science: Calculating pollution levels.
  • Rocketry: Mixing fuel and oxidizer in the perfect ratio.

Criticism / Limitations

Stoichiometry assumes ideal conditions. In reality, side reactions, incomplete reactions, and impurities often reduce the actual yield.

Further Reading

  • Zumdahl, Chemistry
  • Lavoisier, Elements of Chemistry