General Chemistry: Trends, Stoichiometry, and Equilibrium
General chemistry on the PA-CAT is mostly about trends and quantitative relationships. Master atomic structure, electron configuration, and periodic trends: atomic radius decreases across a period and increases down a group; ionization energy and electronegativity move the opposite way. Bonding ranges from ionic through polar covalent to nonpolar covalent based on electronegativity difference. Know VSEPR shapes and how lone pairs distort bond angles.
Stoichiometry depends on the limiting reagent and on unit discipline. Convert mass to moles, use the balanced equation, then convert back. For solutions, molarity is moles per liter; dilution uses M1V1 = M2V2. Equilibrium follows Le Chatelier: changing concentration, pressure, or temperature shifts the system; adding a catalyst changes only the rate. Acids and bases: strong acids dissociate completely; buffers work best near pKa, per Henderson–Hasselbalch. Thermodynamics separates spontaneity (ΔG) from speed (kinetics), and ΔG = ΔH − TΔS.
Traps: assuming the most abundant reactant is limiting; confusing pH with pKa; forgetting that heat can shift equilibrium without changing K for concentration changes; and misapplying gas laws when temperature is not in kelvin.
Worked example: 4 g of H2 (2 mol) reacts with 32 g of O2 (1 mol) to form water. The equation 2H2 + O2 → 2H2O consumes H2 and O2 in a 2:1 ratio, so neither is limiting here; 2 mol of water forms, about 36 g.
Rule of thumb: convert to moles first, identify the limiting reagent, then calculate products from that reagent only.