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Stoichiometry Basics and Calculations

Sep 10, 2025

Overview

This lecture covers stoichiometric calculations, including interpreting balanced chemical equations, converting between mass, moles, and molecules, and applying these skills to solve quantitative chemical reaction problems.

Interpreting Balanced Chemical Equations

  • Balanced equations show the ratio of molecules or moles of reactants and products.
  • Example: 2H₂ + O₂ → 2H₂O means 2 molecules (or moles) of hydrogen react with 1 molecule (or mole) of oxygen to produce 2 molecules (or moles) of water.
  • Coefficients in balanced equations help predict reactant and product amounts.

Calculations Using Molecule Ratios

  • Use coefficients as conversion factors between molecules of substances.
  • To find required molecules of H₂ to make 2.4 × 10³ H₂O molecules: use 2:2 ratio (H₂O:H₂) → need 2.4 × 10³ molecules H₂.
  • For O₂, use a 2:1 ratio (H₂O:O₂) → need 1.2 × 10³ molecules O₂.

Calculations Using Mole Ratios

  • Coefficients also express mole ratios for conversion.
  • For 0.89 moles O₂, the 1:2 ratio (O₂:H₂O) gives 1.8 moles H₂O produced.

Mass-Mass Stoichiometry Calculations

  • To find how many grams of H₂ react with 1.57 g of O₂:
    • Convert grams O₂ to moles using molar mass (32 g/mol).
    • Use mole ratio (1 O₂ : 2 H₂) to find moles H₂.
    • Convert moles H₂ to grams (2 g/mol), yielding 0.196 g H₂.

Multi-Step Stoichiometry Problems

  • Problems may start or end with mass, moles, or molecules.
  • Example: Given 1.8 moles O₂, calculate H₂O molecules:
    • Convert moles O₂ to moles H₂O (1:2 ratio).
    • Convert moles H₂O to molecules using Avogadro's number.
    • Result: 2.2 × 10²⁴ molecules H₂O.
  • Example: Given 1.25 × 10²⁴ H₂O molecules, find grams H₂:
    • Convert molecules H₂O to moles, use 2:2 ratio (H₂O:H₂) to get moles H₂, then convert to grams: 4.15 g H₂.

Key Terms & Definitions

  • Stoichiometry — Calculation of reactant and product quantities in a chemical reaction.
  • Coefficient — Number in a chemical equation showing the ratio of molecules or moles.
  • Molar Mass — Mass of one mole of a substance.
  • Avogadro's Number — 6.02 × 10²³ particles/mole.

Action Items / Next Steps

  • Practice stoichiometric calculations involving mass, moles, and molecules.
  • Review conversion flow chart for mass-mole-molecule relationships.
  • Complete assigned stoichiometry exercises from textbook or course materials.

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