Chemistry: Particles, Bonding, Reactions, and Stoichiometry
Chemistry in the Chiba Entrance Science exam tests atomic structure, bonding, reaction types, stoichiometry, acids and bases, redox, and safe interpretation of laboratory observations. Numerical items usually reduce to the mole concept and a balanced equation.
Core rules
- Balance equations by conserving each element; coefficients give mole ratios, not mass ratios.
- n = m/M relates amount, mass, and molar mass; for solutions, n = cV with volume in liters.
- Ionic compounds dissociate in water; strong acids and bases ionize essentially completely, while weak ones do so partially.
- pH = -log[H+] for dilute aqueous solutions; each pH unit represents a tenfold change in hydrogen-ion concentration.
- In redox, oxidation is loss of electrons and reduction is gain; balance charge as well as atoms.
Worked example
What volume of 0.50 mol/L HCl is needed to neutralize 4.0 g of NaOH? The molar mass of NaOH is 40 g/mol, so n(NaOH) = 4.0/40 = 0.10 mol. The balanced equation is NaOH + HCl → NaCl + H2O, a 1:1 mole ratio. Therefore 0.10 mol HCl is required. From c = n/V, V = n/c = 0.10/0.50 = 0.20 L, or 200 mL.
Traps that cost marks
- Using an unbalanced equation to form mole ratios.
- Forgetting to identify the limiting reagent when two reactant amounts are given.
- Confusing concentration with amount of substance.
- Treating a weak acid as fully ionized.
- Ignoring significant figures or reporting a volume in milliliters when liters are required.