Chemical Reactions and Stoichiometry
This domain checks whether you can classify and balance reactions, write net ionic equations, convert among mass, moles, and particles, and use mole ratios to find limiting reactants and percent yield.
Reaction types
- Synthesis: A + B → AB. Decomposition: AB → A + B.
- Single replacement: A + BC → AC + B, which occurs only if A is more active than B on the activity series.
- Double replacement: AB + CD → AD + CB, driven by formation of a precipitate, a gas, or water.
- Combustion of a hydrocarbon yields CO₂ and H₂O.
Net ionic equations
Dissociate soluble strong electrolytes written (aq), keep solids, liquids, gases, and weak electrolytes intact, then cancel spectator ions. For AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq), the net equation is Ag⁺(aq) + Cl⁻(aq) → AgCl(s).
Mole calculations
- Moles = mass ÷ molar mass; particles = moles × 6.02 × 10²³.
- Molarity = moles of solute ÷ liters of solution; dilution follows M₁V₁ = M₂V₂.
- Percent composition = mass of element ÷ molar mass × 100.
- Empirical formula is the simplest ratio; molecular formula = empirical formula × a whole number found from the molar mass.
Worked example
For 2H₂ + O₂ → 2H₂O, 4.0 g H₂ is 2.0 mol and 32 g O₂ is 1.0 mol. The ratio needs 2 mol H₂ per 1 mol O₂, so both are consumed exactly, giving 2.0 mol H₂O, or 36 g.
Traps
- Skipping the balance before using mole ratios.
- Feeding grams directly into a mole ratio.
- Counting spectator ions in a net ionic equation.
- Using solvent volume instead of total solution volume for molarity.