General Chemistry
General Chemistry on the OAT tests stoichiometry, atomic structure, bonding, gases, solutions, equilibrium, acids and bases, redox, thermodynamics, kinetics, and electrochemistry. Most items combine a chemical principle with a short calculation.
Core rules
- Convert mass to moles using molar mass; use mole ratios from the balanced equation. The limiting reagent controls theoretical yield.
- Ideal gas law: PV = nRT. Temperature must be in kelvin. At constant T and P, equal volumes of gases contain equal moles.
- Equilibrium: compare reaction quotient Q with K. If Q < K, the reaction proceeds forward; if Q > K, it proceeds reverse.
- Weak acid approximation: for HA with concentration C and Ka, x ≈ √(KaC) when x is small compared with C. pH = −log[H+].
- Hess’s law adds enthalpy changes; bond breaking absorbs energy and bond formation releases energy. Reaction rate depends on activation energy, not only ΔG.
Worked example
Find the pH of 0.10 M weak acid with Ka = 1.8 × 10⁻⁵. Let x = [H+]. Ka ≈ x²/C, so x² = (1.8 × 10⁻⁵)(0.10) = 1.8 × 10⁻⁶. Thus x ≈ 1.34 × 10⁻³ M, and pH = −log(1.34 × 10⁻³) ≈ 2.87. The approximation is acceptable because x is about 1.3% of C.
Traps
Do not use grams directly in mole ratios; do not forget limiting reagent; do not confuse pH and pOH; do not assume a shift changes K; do not mix up oxidation and reduction.