Chemistry: Moles, Reactions, and Acids–Bases
Chemistry tests atomic structure and periodic trends, the mole concept, stoichiometry, acid–base behavior, redox, and the functional groups that appear in biological molecules. The exam rewards clean unit handling and balanced equations more than memorized exceptions.
Core rules and concepts
- Protons define the element; neutrons change the isotope; electrons determine charge and bonding. Across a period, atomic radius decreases and electronegativity increases; down a group, radius increases and ionization energy falls.
- One mole contains 6.02 × 10^23 particles. Molar mass converts mass to moles; concentration in mol/L converts volume to moles.
- Balance equations before using mole ratios. For a reaction aA + bB → products, the ratio n(A)/a = n(B)/b at stoichiometric conditions.
- Strong acids and bases ionize completely in water; weak ones only partly. pH = −log[H3O+], so a tenfold change in hydrogen-ion concentration changes pH by one unit.
- Oxidation is loss of electrons; reduction is gain. In organic molecules, recognize alcohols (−OH), aldehydes (−CHO), ketones (C=O), carboxylic acids (−COOH), amines (−NH2), and esters (−COO−).
Worked example
A 20.0 mL sample of hydrochloric acid is neutralized by 25.0 mL of 0.100 M sodium hydroxide. Find the acid concentration. The reaction is HCl + NaOH → NaCl + H2O, a 1:1 ratio. Moles of NaOH = 0.0250 L × 0.100 mol/L = 0.00250 mol. Therefore moles of HCl = 0.00250 mol. Concentration of HCl = 0.00250 mol ÷ 0.0200 L = 0.125 M. Keeping volumes in liters and checking the 1:1 coefficient prevents the common error of multiplying by 20/25 instead of dividing correctly.
Traps
Using milliliters directly in c = n/V is the most frequent error. Forgetting coefficients in non-1:1 reactions, confusing strong with concentrated, and treating pH as a linear scale also cost marks. When calculating oxidation numbers, check that the sum equals the overall charge.