Chemistry: Structure, Stoichiometry, and Equilibrium
What the Chemistry section tests
The Chemistry domain covers atomic structure and periodic trends, chemical bonding, stoichiometry, gases, solutions, acids and bases, equilibrium, thermochemistry, and introductory organic functional groups. The exam favors multi-step calculation and explanation of trends rather than isolated recall.
Core rules
- Periodic trends: atomic radius decreases left to right across a period and increases down a group; ionization energy and electronegativity generally move in the opposite direction.
- Balance equations before using mole ratios. Convert mass to moles with n = m/M, then use the coefficients. For gases at STP, 1 mol occupies 22.4 L; more generally use PV = nRT.
- Molarity is M = n/V in liters; dilution uses M1V1 = M2V2. Strong acids and bases dissociate completely, weak ones only partly, so pH calculations differ.
- Le Châtelier's principle: adding reactant shifts right, adding product shifts left, increasing pressure shifts toward fewer gas moles, and heating shifts in the endothermic direction.
- Bond polarity comes from electronegativity difference; molecular shape comes from VSEPR electron-pair repulsion. Organic functional groups such as alcohols, aldehydes, ketones, carboxylic acids, and esters have distinct suffixes and reactivity.
Worked example
For 2H2 + O2 -> 2H2O, 5.0 g H2 reacts with 32.0 g O2. Moles: H2 = 5.0/2.0 = 2.5 mol; O2 = 32.0/32.0 = 1.0 mol. The ratio needs 2 mol H2 per 1 mol O2, so 1.0 mol O2 consumes 2.0 mol H2. H2 is in excess; O2 is limiting. Water formed = 2(1.0) = 2.0 mol, mass = 2.0 x 18.0 = 36.0 g.
Traps that cost marks
The limiting reagent is not always the one with the smaller mass. Do not ignore coefficients when comparing moles. Watch significant figures and state symbols. In pH, a tenfold change in [H+] changes pH by one unit, not by ten.