Chemistry
What NSEJS chemistry tests
Atomic structure and periodicity, bonding, the mole, acids–bases–salts, metals and carbon compounds. The marks sit in calculation and in explaining trends, not in reciting them.
- Mole: n = m/M = N/N_A = V/22.4 L for a gas at STP, with N_A = 6.022 × 10²³.
- Limiting reagent: divide each reactant's moles by its stoichiometric coefficient; the smallest quotient limits the yield.
- Periodic trends: left to right, nuclear charge grows so radius falls and ionisation energy and electronegativity rise. Top to bottom, radius grows and metallic character increases.
- Bonding: a large electronegativity difference plus a metal–nonmetal pair gives ionic bonding; ionic solids conduct only when molten or dissolved.
- Carbon: tetravalency and catenation; successive members of a homologous series differ by CH₂, that is 14 u.
Worked example. 12 g of carbon is burnt in 32 g of oxygen. For C + O₂ → CO₂, moles of C = 12/12 = 1 and moles of O₂ = 32/32 = 1. The coefficients are 1 : 1, so neither reactant is in excess, 1 mol of CO₂ forms and its mass is 44 g. Had only 16 g of O₂ (0.5 mol) been supplied, oxygen would limit: 0.5 mol CO₂ = 22 g, and 6 g of carbon would remain unburnt.
Traps. Comparing masses instead of moles; using M = 16 for oxygen gas; treating a weak acid as fully dissociated when computing pH; confusing atomic mass with molecular mass; writing CO₂ as 12 + 16; taking the mole ratio from the formula rather than the balanced equation.