Chemistry - Structure, Stoichiometry and Acid–Base Calculations
Chemistry in the ENP Alger access contest is the shortest but most detail-heavy part of the paper. It tests atomic structure and bonding, quantitative stoichiometry in solution, acid–base equilibria, kinetics, and a small organic component centred on functional groups and reaction types. Most questions are short calculations or justifications, so precision in units and in the statement of assumptions matters as much as the arithmetic.
Core content
- Electronic configuration and periodic trends: atomic radius, ionisation energy, electronegativity; note the half-filled and fully filled exceptions such as Cr and Cu.
- Bonding: ionic, covalent and metallic; VSEPR geometries; polarity from geometry plus electronegativity difference.
- Quantitative work: mole, molarity, dilution, limiting reagent, percentage yield.
- Acid–base: strong versus weak, Ka and pKa, pH, titration equivalence point.
- Kinetics and equilibrium: rate laws, factors affecting rate, Le Chatelier's principle, Kc and Kp.
- Organic: functional groups, nomenclature, addition, substitution and elimination, isomerism.
Worked example
Calculate the pH of 0.10 mol/L ethanoic acid with Ka = 1.8×10⁻⁵. CH₃COOH + H₂O ⇌ CH₃COO⁻ + H₃O⁺, so with x = [H₃O⁺], Ka = x²/(0.10 − x). Assume x ≪ 0.10: x² = 1.8×10⁻⁶, giving x = 1.34×10⁻³ mol/L and pH = 2.87. Then check the assumption: x/0.10 = 1.3 %, below the usual 5 % threshold, so the approximation is justified and the answer stands. Without the check, the result is only an assertion.
Traps that cost marks
- Dilution changes the volume, not the number of moles: compute n first, then divide by the new volume.
- pH significant figures are in the decimals - pH = 2.87 carries two.
- Confusing Ka with pKa, and forgetting that a smaller Ka means a weaker acid.
- Using [H₃O⁺] = c for a weak acid; that identity holds only for strong acids.
- An unbalanced equation, or a limiting reagent assumed rather than calculated.
- Omitting state symbols or the expected ionic form where the question asks for an equation.