Most formula sheets are a wall of symbols. That is fine in an exam, where you already know what each line is for, and useless the week before, when the question is which line to reach for. The guides below are organized the way the decision actually goes: what the relationship says, what has to be true before you can use it, and a worked example with real numbers so you can see the method, not just the result. Each guide lists the open textbook pages it was checked against.

Stoichiometry

Moles, masses, yields and the ratios that connect them. The arithmetic every chemistry course runs on.

Formula	Says
Empirical formula `%X = (mass X / mass compound) × 100%`	An empirical formula gives the simplest whole-number ratio of atoms in a compound.
Percent yield formula `percent yield = (actual yield / theoretical yield) × 100%`	Percent yield compares what a reaction actually produced with the most it could have produced.
Limiting reactant `compare n(X) / coefficient(X) across reactants; the smallest value is limiting`	The limiting reactant is the one that runs out first and therefore caps how much product forms.
Molar mass `M = Σ (atoms of each element × atomic mass)`	Molar mass is the mass of one mole of a substance in grams per mole, and it equals the sum of the atomic masses in the formula.
Molecular formula `n = molar mass / empirical formula mass`	A molecular formula gives the actual number of each kind of atom in one molecule, where the empirical formula gives only the simplest ratio.

Atomic structure and bonding

Where the electrons are and how atoms hold together.

Formula	Says
Electron configuration `1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f 6d 7p`	An electron configuration lists which orbitals an atom's electrons occupy, written as subshell labels with electron counts.
Lewis structure `1 count valence electrons, 2 connect atoms with single bonds, 3 complete octets on outer atoms, 4 put leftovers on the central atom, 5 form multiple bonds if the central atom is short`	A Lewis structure is a drawing that shows where every valence electron sits in a molecule or ion, as bonding pairs between atoms and lone pairs on them.
Covalent bond `ΔEN = \|EN(A) - EN(B)\|`	A covalent bond is a shared pair of electrons holding two atoms together.

Solutions, acids and bases

Concentration, dilution, pH and buffers.

Formula	Says
pH formula `pH = -log[H₃O⁺]`	pH is the negative base-10 logarithm of the hydronium ion concentration, so it compresses a huge concentration range onto a readable scale.
Titration `n(titrant) = M(titrant) × V(titrant)`	A titration finds an unknown concentration by adding a solution of known concentration until the reaction is exactly complete.
Henderson-Hasselbalch equation `pH = pKa + log([A⁻] / [HA])`	The Henderson-Hasselbalch equation gives the pH of a buffer from the pKa of its weak acid and the ratio of conjugate base to acid.
Dilution formula `C₁V₁ = C₂V₂, often written M₁V₁ = M₂V₂`	The dilution formula says concentration times volume stays constant when you add solvent, because the amount of solute never changes.
Molarity formula `M = mol solute / L solution`	Molarity is moles of solute per liter of solution, the concentration unit most chemistry calculations run on.

Thermodynamics and kinetics

Whether a reaction goes, and how fast.

Formula	Says
Nernst equation `E_cell = E°_cell - (RT / nF)·ln Q`	The Nernst equation gives the cell potential when concentrations are not all standard.
Hess's law `ΔH_reaction = ΣΔH_steps`	Hess's law says the enthalpy change of a reaction depends only on where it starts and ends, not on the route taken.
Half-life formula `t½ = ln 2 / k ≈ 0.693 / k`	Half-life is the time for half the material present to react or decay.
Rate law `rate = k[A]^m[B]^n`	A rate law states how a reaction's rate depends on the concentrations of its reactants, as a rate constant multiplied by concentrations raised to powers.
Arrhenius equation `k = A·e^(-Ea / RT)`	The Arrhenius equation describes how a rate constant grows with temperature.
Gibbs free energy equation `G = H - TS`	The Gibbs free energy change combines enthalpy and entropy into one test for spontaneity at constant temperature and pressure.

Gases and equilibrium

The gas laws and the balance point of a reversible reaction.

Formula	Says
Ideal gas law `PV = nRT`	The ideal gas law ties pressure, volume, amount and temperature into one relation, PV = nRT.

How to use a formula sheet well

Start from the quantities you know and the one you want, then find the formula that contains exactly those and nothing you would have to guess. Check its conditions before the algebra, not after: an ideal gas, a dilute solution, a reaction at equilibrium. Write the units through every line, because a unit that does not cancel is the fastest way to catch a wrong formula. And keep one worked example per formula that you can reproduce from memory; that is what turns the sheet into a method.