Chemistry
What this domain tests
Chemistry questions cover atomic structure and periodic trends, bonding, reactions, stoichiometry, solutions, acids and bases, and basic thermodynamics and kinetics.
Atomic structure and periodicity
Atoms are defined by proton number; isotopes differ in neutrons. Electron configurations fill lower-energy orbitals first. Across a period, increasing nuclear charge pulls electrons closer, so atomic radius decreases and ionization energy and electronegativity generally increase. Down a group, added shells and shielding increase radius and decrease ionization energy.
Bonding and reactions
Ionic bonds transfer electrons between metals and nonmetals; covalent bonds share electrons. Use Lewis structures and electronegativity differences to judge polarity. Balance equations to conserve atoms and charge. Recognize synthesis, decomposition, single replacement, double replacement, and combustion.
Stoichiometry and solutions
The mole connects mass and particle number: n = m/M. Use mole ratios from balanced equations. The limiting reactant runs out first and determines product yield. Molarity = moles solute / liters solution; dilution: M1V1 = M2V2. pH = -log[H3O+].
Worked example
3.0 g H2 reacts with 16.0 g O2: 2H2 + O2 -> 2H2O. Find limiting reactant and mass of water. n(H2) = 3.0 g / 2.02 g/mol = 1.49 mol. n(O2) = 16.0 g / 32.00 g/mol = 0.500 mol. Required H2 for 0.500 mol O2 is 1.00 mol, so O2 is limiting. Water = 2 x 0.500 = 1.00 mol; mass = 1.00 x 18.02 = 18.0 g. H2 remains.
Traps
Do not use grams directly in mole ratios. Do not ignore coefficients when balancing. Do not confuse strong/weak acids with concentrated/dilute solutions. Watch significant figures and units.