Stoichiometry and Chemical Reactions
Scope
The exam checks mole–mass–volume conversions, limiting reagent logic, percent yield, empirical and molecular formulas, solution concentration and dilution, net ionic equations, and redox balancing.
Core rules
- Convert every given quantity to moles before comparing; molar mass is the bridge, and for gases at STP 22.4 L/mol is the shortcut.
- Limiting reagent: divide moles of each reactant by its stoichiometric coefficient; the smallest quotient limits.
- Percent yield = (actual/theoretical) × 100, where theoretical comes from the limiting reagent.
- Molarity M = mol solute / L solution; dilution obeys M₁V₁ = M₂V₂.
- For net ionic equations, keep only species that change; spectator ions cancel.
- Oxidation numbers track electron transfer: assign 0 to elements, −2 to O (except peroxides), +1 to H (except hydrides), and let the sum equal the overall charge.
Worked example
10.0 g Al reacts with 10.0 g Cl₂ to form AlCl₃. Moles: Al = 10.0/26.98 = 0.371 mol; Cl₂ = 10.0/70.90 = 0.141 mol. For 2Al + 3Cl₂ → 2AlCl₃, 0.371 mol Al needs 0.557 mol Cl₂, but only 0.141 mol is present, so Cl₂ limits. AlCl₃ formed = 0.141 × (2/3) = 0.0940 mol; mass = 0.0940 × 133.34 = 12.5 g. If 10.8 g were isolated, percent yield = 10.8/12.5 × 100 = 86.4%.
Traps
- Comparing reactant masses directly instead of moles, which reverses the limiting reagent.
- Dropping waters of hydration from the molar mass of a hydrate.
- Reporting percent yield above 100% as success when the product is wet or impure.
- Forgetting to balance charge when balancing redox in acidic versus basic media.