Overview
This lecture covers core stoichiometry concepts using the mole concept, including how to relate mass, volume, and moles in chemical equations, and step-by-step solutions for typical exam questions.
Mole Concept Formulas
- Number of moles = Mass given / Molar mass.
- Number of moles = Volume at STP / 22.4 L (molar volume at STP).
Mass-Mole Calculations
- To find moles: divide the mass of the substance by its molar mass.
- Example: 32 g methane (CH₄) → 32/16 = 2 moles.
- Example: 49 g H₂SO₄ → 49/98 = 0.5 moles.
- To find mass: multiply number of moles by molar mass.
- Example: 2 moles SO₂ → 2 × 64 g = 128 g.
Mole-Volume Relation at STP
- Molar volume at STP is always 22.4 L for gases.
- Volume = Number of moles × 22.4 L.
- Example: 3 moles CO₂ → 3 × 22.4 = 67.2 L.
- Example: 0.5 mole NO → 0.5 × 22.4 = 11.2 L.
Stoichiometry with Chemical Equations
- Ensure equations are balanced before calculations.
- Coefficients represent mole ratios.
- Example: CH₄ + 2O₂ → CO₂ + 2H₂O shows 1 mole CH₄ forms 1 mole CO₂.
- If 16 g CH₄ is burned, moles = 1, so 1 mole CO₂ produced (44 g).
Solving Stoichiometry Problems
- Step 1: Convert given mass or volume to moles using the appropriate formula.
- Step 2: Use balanced equation to find mole ratio between substances.
- Step 3: Convert moles of product/reactant to mass or volume as required.
Advanced Examples
- Find mass, volume or moles for any reactant or product using mole ratios from balanced equations.
- If given volume at STP, always divide by 22.4 L to find moles.
- For complex equations, break down steps: find moles, apply the mole ratio, then find the required mass or volume.
Key Terms & Definitions
- Mole — the amount of substance containing Avogadro's number (6.022×10²³) of particles.
- Molar Mass — mass of one mole of a substance (g/mol).
- Molar Volume — volume occupied by one mole of a gas at STP (22.4 L).
- Stoichiometry — calculation of reactants and products in chemical reactions using the mole concept.
- Balanced Equation — chemical equation with equal numbers of each atom on both sides.
Action Items / Next Steps
- Practice 3-5 similar stoichiometry word problems using the outlined steps.
- Review balancing chemical equations and calculating molar masses for assigned compounds.