The mole is chemistry's counting unit — it bridges the gap between atoms we can't see and grams we can measure. These three relationships let you convert freely between mass, amount of substance, number of particles, and solution concentration.
n = m / MW • N = n × NA • M = n / V
Stoichiometry is the quantitative bookkeeping of chemical reactions. A balanced equation tells you the exact mole ratios of reactants consumed and products formed, letting you predict the theoretical yield and identify the limiting reagent.
Balance first, then use coefficients as mole ratios to find the limiting reagent and theoretical yield.
Redox reactions involve the transfer of electrons between species. Tracking oxidation states reveals which atom is oxidised (loses electrons) and which is reduced (gains electrons) — essential for understanding batteries, corrosion, and metabolism.
OIL RIG — Oxidation Is Loss, Reduction Is Gain (of electrons). Compare oxidation states before and after.
Solutions are homogeneous mixtures described by concentration. Molarity (M) tells you how many moles of solute are dissolved per litre of solution, and the dilution equation lets you calculate the new concentration when you add solvent.
M = n / V • M1V1 = M2V2. Leave one field blank to solve.
The Ideal Gas Law relates pressure, volume, temperature, and amount of gas in a single equation. It assumes gas particles have negligible volume and no intermolecular forces — a good approximation at moderate temperatures and low pressures.
PV = nRT (R = 0.08206 L·atm/mol·K). Leave one variable blank.
Thermodynamics describes the energy changes in chemical reactions. The sign of the enthalpy change (ΔH) tells you whether a reaction releases energy to the surroundings (exothermic) or absorbs energy from the surroundings (endothermic).
Exothermic (ΔH < 0): system releases heat. Endothermic (ΔH > 0): system absorbs heat.
ΔH < 0 — products lower in energy
ΔH > 0 — products higher in energy
Reaction kinetics studies how fast reactions proceed and what factors influence the rate. Temperature increases molecular speed, catalysts lower the activation energy barrier, and higher concentrations mean more frequent collisions between reactant molecules.
Average rate = |ΔC| / Δt. Higher temperature, catalysts, and higher concentration all tend to increase rate.
Chemical equilibrium is reached when the forward and reverse reaction rates are equal, so concentrations stop changing. The equilibrium constant K quantifies the ratio of products to reactants at equilibrium, and Le Chatelier's principle predicts how the system shifts when disturbed.
K = [P]c / ([A]a[B]b). K > 1 favors products; K < 1 favors reactants.
Atomic structure and polarity govern how atoms bond. Atomic radius shrinks across a period as nuclear charge increases, while electronegativity — the tendency to attract shared electrons — determines whether a bond is nonpolar covalent, polar covalent, or ionic.
Atomic radius decreases across a period, increases down a group. Electronegativity difference determines bond polarity.