Chemistry: Moles, Equilibrium and Acid–Base Calculations
Akita chemistry items test quantitative reasoning across stoichiometry, equilibrium, acid–base behaviour and redox, with periodic trends as supporting knowledge. Most marks come from correct mole arithmetic and clearly stated reasoning rather than memorized colours or trivia.
Core rules and concepts
- n = m/M, and concentration c = n/V in mol/L; convert mL to L before substituting.
- Balance the equation first; coefficients give the mole ratio.
- Limiting reagent: divide moles of each reactant by its coefficient; the smallest quotient limits the reaction.
- Le Chatelier's principle: a system at equilibrium shifts to oppose the imposed change. K depends on temperature only.
- For a monoprotic acid–base titration, moles H⁺ = moles OH⁻ at the equivalence point.
Worked example
4.0 g of H₂ reacts with 32 g of O₂. What mass of water forms? (H = 1.0, O = 16)
Reasoning: n(H₂) = 4.0/2.0 = 2.0 mol; n(O₂) = 32/32 = 1.0 mol. The equation 2H₂ + O₂ → 2H₂O requires H₂:O₂ = 2:1, so 2.0 mol H₂ needs exactly 1.0 mol O₂. Neither reactant is in excess. Water formed = 2.0 mol × 18 g/mol = 36 g.
Traps that cost marks
- Treating H₂ and O₂ as monatomic (M = 1 and 16).
- Using a mass ratio where a mole ratio is required.
- Forgetting to convert mL to L in titration calculations.
- Claiming a catalyst changes the equilibrium constant.