Chemistry: Stoichiometry, Solutions, and Periodic Trends
Chemistry questions on this exam emphasize quantitative reasoning grounded in a few core models. Master the mole concept: Avogadro's number, molar mass from the periodic table, and the bridge between grams, moles, and particles. Stoichiometry depends on balanced equations and mole ratios; limiting-reactant problems require comparing the moles of each reactant divided by its coefficient. Solution chemistry involves molarity (moles solute per liter solution), dilution (M1V1 = M2V2), and percent composition by mass.
Periodic trends are frequent: atomic radius decreases across a period and increases down a group; ionization energy and electronegativity increase across a period and decrease down a group. Know that ionic compounds form between metals and nonmetals, covalent bonds share electrons, and acids donate protons. Gas behavior follows the ideal gas law, PV = nRT, with temperature in kelvin.
Traps include using grams instead of moles in mole ratios, forgetting to balance equations before applying ratios, mixing up molarity with molality, and ignoring significant figures. Temperature conversions are a common error: add 273.15 for kelvin. In limiting-reactant problems, the reactant with the smaller mole-to-coefficient ratio limits the product, not necessarily the one with fewer grams.
Rule of thumb: convert everything to moles first, then use the balanced equation, then convert back to the requested unit.
Worked example: How many grams of water form when 4.0 g of H2 reacts with excess O2? Moles H2 = 4.0 / 2.02 = 1.98 mol. From 2H2 + O2 -> 2H2O, the ratio is 1:1, giving 1.98 mol H2O. Mass = 1.98 x 18.02 = 35.7 g.