Chemistry
Core ideas
Chemistry questions combine structure with calculation. Know atomic number, mass number, isotopes, electron configuration, periodic trends, and the difference between ionic and covalent bonding. For stoichiometry, balance the equation first, convert mass to moles using molar mass, use the mole ratio, and convert back to the requested quantity. In solutions, distinguish solute, solvent, and concentration; use c = n/V and the dilution relation c₁V₁ = c₂V₂. Review acids and bases, pH as a logarithmic scale, oxidation states, and simple solubility rules. In inorganic chemistry, link formulas to names and charges, especially for common acids, bases, and salts.
Traps
Traps include failing to balance equations, treating the limiting reagent as the reactant with the smallest mass rather than the smallest mole ratio, mixing g/L with mol/L, and forgetting that dilution changes volume as well as concentration. Do not assume that a strong acid is always concentrated, and do not ignore state symbols when they are given.
Worked example
10.0 g of CaCO₃ (molar mass 100 g/mol) decomposes completely: CaCO₃ → CaO + CO₂. Moles CaCO₃ = 10.0/100 = 0.100 mol, so CO₂ = 0.100 mol and mass CO₂ = 0.100 × 44 = 4.4 g. Rule of thumb: every stoichiometry problem goes mass → moles → mole ratio → moles → mass.