Chemical Reactions, Stoichiometry, and Solution Chemistry
This domain tests quantitative reaction chemistry: balancing equations, the mole concept, limiting reactants, solution concentration, and the ionic and redox language used to describe reactions.
Core rules
- Balance by changing coefficients only; never change subscripts. Check atoms and charge.
- Convert mass to moles with molar mass, apply the balanced mole ratio, then convert back to mass or concentration.
- Identify the limiting reactant by comparing the mole ratio actually available with the stoichiometric ratio required.
- Molarity M = mol solute / L solution; dilution uses M1V1 = M2V2.
- Solubility rules predict precipitates. Write complete ionic and net ionic equations, canceling spectator ions.
- In redox, assign oxidation numbers, separate oxidation and reduction half-reactions, and balance charge with electrons.
Worked example
10.0 g Al and 10.0 g Cl2 react: 2Al + 3Cl2 -> 2AlCl3. Moles Al = 10.0 / 26.98 = 0.371 mol; moles Cl2 = 10.0 / 70.90 = 0.141 mol. Required Cl2 for all Al = 0.371 x 3/2 = 0.556 mol, so Cl2 is limiting. Moles AlCl3 = 0.141 x 2/3 = 0.0940 mol. Mass AlCl3 = 0.0940 x 133.33 = 12.5 g. Percent yield uses actual yield divided by theoretical yield, times 100.
Traps that cost marks
- Comparing gram amounts instead of moles when finding the limiting reactant.
- Using the product coefficient in the limiting-reactant ratio.
- Forgetting that soluble strong electrolytes are written as separated ions.
- Confusing oxidation (loss of electrons) with reduction (gain of electrons).