Chemistry: Moles, Equilibrium, and Functional Groups
EJU Chemistry tests atomic structure and bonding, the mole and solutions, chemical equilibrium, acids and bases, redox and electrochemistry, and organic and inorganic chemistry. Quantitative items usually reduce to a mole relationship or an equilibrium expression.
Core rules and concepts
- Amount of substance: n=m/M; for a gas at 0°C and 1 atm, n=V/22.4; concentration c=n/V.
- Bonding: ionic, covalent, and metallic bonds; electronegativity differences predict bond polarity; intermolecular forces affect boiling point.
- Equilibrium: Le Chatelier's principle; K excludes pure solids and liquids.
- Acid-base: pH=-log[H⁺]; strong acids dissociate completely; weak acids need Ka.
- Redox: assign oxidation numbers; oxidation occurs at the anode; E°cell=E°cathode−E°anode.
- Organic: identify functional groups and isomer type; addition versus substitution depends on saturation.
Worked example
Find the pH of 0.10 mol/L acetic acid, Ka=1.8×10⁻⁵. Let x=[H⁺]. Ka≈x²/c, so x=√(1.8×10⁻⁵×0.10)=√(1.8×10⁻⁶)=1.34×10⁻³ mol/L. pH=−log(1.34×10⁻³)=2.87. The approximation x≪c is valid here; otherwise solve the quadratic.
Traps that cost marks
Using gas volume without checking 0°C/1 atm; ignoring the limiting reagent; writing K with a solid included; confusing anode and cathode in a cell; treating a weak acid as fully dissociated; missing cis-trans or optical isomerism.