Chemical Reactions and Stoichiometry
Balancing and reaction types
Balance equations by changing coefficients, never subscripts. Common types: synthesis, decomposition, single replacement, double replacement (precipitation/neutralization), and combustion. Solubility rules and activity series predict whether a replacement occurs. Ionic equations show dissociated strong electrolytes; spectators are removed.
Mole calculations
Molar mass converts grams to moles. Avogadro's number (6.022 × 10^23 particles/mol) converts moles to particles; molar volume 22.4 L/mol applies to ideal gases at STP (0 °C, 1 atm). Use mole ratios from the balanced equation. For solutions, moles = M × V(L).
Limiting reactant and yield
Compute moles of each reactant, divide by its coefficient, and choose the smaller quotient. That reactant limits. Theoretical yield comes from the limiting reactant; percent yield = actual/theoretical × 100%.
Worked example
For 2H2 + O2 → 2H2O, 4.0 g H2 is 1.98 mol; 32.0 g O2 is 1.00 mol. Dividing by coefficients gives H2: 0.99 and O2: 1.00, so H2 is limiting. The equation gives 1.98 mol H2O, mass = 1.98 × 18.02 = 35.7 g. O2 needed = 0.99 mol = 31.7 g, so about 0.3 g O2 remains.
Traps
Using the reactant with larger mass as excess without mole ratios; balancing with subscripts; forgetting to convert mL to L; applying 22.4 L/mol at non-STP conditions; and using the actual yield as the limiting-reactant basis.