Chemical Reactions and Stoichiometry
Chemical Reactions and Stoichiometry
This domain tests balanced equations, reaction classes, mole conversions, limiting reactants, yields, and solution stoichiometry. Most items reward a clear mole map and careful unit tracking.
Core rules
- Balance equations by changing coefficients only; subscripts are fixed.
- Convert mass to moles with molar mass, use mole ratios from the balanced equation, then convert back to mass or volume.
- Identify the limiting reactant by comparing the mole ratio actually available with the ratio required. The limiting reactant controls theoretical yield.
- Percent yield = actual yield / theoretical yield x 100. For solutions, moles = molarity x liters.
Reaction classes
Synthesis, decomposition, single replacement, double replacement, combustion, and acid-base neutralization. In net ionic equations, spectator ions are removed.
Worked example
For 2 H2 + O2 -> 2 H2O, 10.0 g H2 is 4.96 mol and 10.0 g O2 is 0.3125 mol. Required H2:O2 ratio is 2:1, so 0.3125 mol O2 needs 0.625 mol H2; H2 is in excess and O2 is limiting. Theoretical water = 2 x 0.3125 = 0.625 mol. Mass = 0.625 x 18.02 = 11.3 g. If 9.50 g is collected, percent yield = 9.50/11.3 x 100 = 84.1 percent.
Traps
Using an unbalanced equation; choosing the reactant with the smaller mass rather than fewer moles; computing yield from the excess reactant; mixing up actual and theoretical yield; and forgetting to convert milliliters to liters.