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AP Chemistry — Equations, Constants & Periodic Table

7 sections · 55 entries · print it and keep it beside your practice sets

On exam day you get a periodic table and a two-page equations sheet for both the multiple-choice and free-response sections. Note what is NOT on it: solubility rules, strong acids, polyatomic ions, and the activity series. Those you must know cold — everything below you can look up.

Practise with the sheet, not from memory. The College Board hands out its own version of this page on exam day, so nothing here is worth memorising for its own sake. What earns points is speed: knowing which section a quantity lives in, and reading off the right line without breaking your train of thought. Keep this open (or printed) for every practice set you do.

Periodic table

Quantity / relationshipWhat it means & how to use it

Periodic table

The exam supplies a full periodic table with atomic number, symbol, and atomic mass to four significant figures for every element — but no names, no electronegativities, and no ion charges.

Practise with our interactive version at /labs/periodic-table, which adds electron configurations and category filters. On the real exam you get masses only, so rehearse molar-mass arithmetic from the table alone.

Molar mass

M = Σ (atomic mass × subscript) g/mol

Sum of atomic masses over the formula

n = m / M

Moles from mass

Particles

N = n N_A

Moles to particles

Atomic structure & spectroscopy

Quantity / relationshipWhat it means & how to use it

E = h ν = h c / λ

Energy of a photon

c = λ ν

Wave relationship for light

A = ε b c

Beer–Lambert law: absorbance from molar absorptivity ε, path length b, concentration c

Gases, liquids & solutions

Quantity / relationshipWhat it means & how to use it

PV = nRT

P V = n R T

Ideal gas law

T must be in kelvin. Match R to your pressure units.

P_A

P_A = X_A × P_total, P_total = Σ P_i

Dalton’s law: partial pressure from mole fraction X_A

X_A

X_A = n_A / n_total

Mole fraction

D = m/V = P M / (R T)

Density of a gas

u_rms

u_rms = √(3 R T / M)

Root-mean-square speed; M in kg/mol

KE_avg = (3/2) k_B T

Average kinetic energy per molecule

M (molarity)

M = moles solute / L solution

Concentration

Dilution

M₁ V₁ = M₂ V₂

Diluting a stock solution

K = °C + 273.15

Temperature conversion

Thermochemistry & thermodynamics

Quantity / relationshipWhat it means & how to use it

q = m c ΔT

Heat absorbed or released; c is specific heat capacity

ΔH°_rxn

ΔH°_rxn = Σ ΔH°_f(products) − Σ ΔH°_f(reactants)

Standard enthalpy from formation enthalpies

ΔS°_rxn

ΔS°_rxn = Σ S°(products) − Σ S°(reactants)

Standard entropy change

ΔG°_rxn

ΔG°_rxn = Σ ΔG°_f(products) − Σ ΔG°_f(reactants)

Standard free energy from formation values

ΔG°

ΔG° = ΔH° − T ΔS°

Gibbs free energy; T in kelvin

ΔG < 0 means thermodynamically favourable. Says nothing about rate.

ΔG° and K

ΔG° = − R T ln K

Linking free energy to the equilibrium constant

ΔG° and E°

ΔG° = − n F E°_cell

Linking free energy to cell potential; n = mol e⁻ transferred

Equilibrium

Quantity / relationshipWhat it means & how to use it

K_c

K_c = ([C]^c [D]^d) / ([A]^a [B]^b)

For a A + b B ⇌ c C + d D, in concentrations

K_p

K_p = (P_C^c P_D^d) / (P_A^a P_B^b)

Same reaction in partial pressures

Q < K: shifts right; Q > K: shifts left; Q = K: at equilibrium

Reaction quotient — same expression, non-equilibrium values

K_a

K_a = [H⁺][A⁻] / [HA]

Weak acid ionization, HA ⇌ H⁺ + A⁻

K_b

K_b = [HB⁺][OH⁻] / [B]

Weak base ionization, B + H₂O ⇌ HB⁺ + OH⁻

K_w

K_w = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴ = K_a × K_b

Autoionization of water at 25 °C

pH, pOH

pH = − log[H⁺], pOH = − log[OH⁻]

Logarithmic scales

pH + pOH

pH + pOH = 14.00

At 25 °C

pK_a = − log K_a, pK_b = − log K_b, pK_a + pK_b = 14.00

Log form of the constants

Henderson–Hasselbalch

pH = pK_a + log([A⁻]/[HA])

pH of a buffer

Buffer is most resistant when [A⁻] = [HA], i.e. pH = pK_a.

K_sp

K_sp = [A]^x [B]^y

Solubility product, e.g. for A_xB_y(s) ⇌ x A + y B

Kinetics

Quantity / relationshipWhat it means & how to use it

Rate

rate = k [A]^m [B]^n

Rate law for order m in A, n in B

Zero order

[A]_t = [A]₀ − k t

Integrated rate law; straight line for [A] vs t

First order

ln[A]_t = ln[A]₀ − k t

Straight line for ln[A] vs t

Second order

1/[A]_t = 1/[A]₀ + k t

Straight line for 1/[A] vs t

t½

t½ = 0.693 / k

Half-life of a first-order reaction

Constant half-life is the signature of first order — only first order has this property.

k(T)

k = A e^(−E_a/RT), ln k = ln A − E_a/(R T)

Arrhenius equation; E_a is activation energy

Electrochemistry

Quantity / relationshipWhat it means & how to use it

E°_cell

E°_cell = E°_cathode − E°_anode

Standard cell potential from reduction potentials

Both taken as reduction potentials. A galvanic cell has E°_cell > 0.

I = q / t

Current as charge per second

Electrolysis

mol e⁻ = q / F = I t / F

Moles of electrons from charge passed

ΔG°

ΔG° = − n F E°_cell

Free energy of a cell

# Constants & conversions

R = 8.314 J/(mol·K)

Universal gas constant (energy units)

R = 0.08206 L·atm/(mol·K) = 62.36 L·torr/(mol·K)

Gas constant (gas-law units)

F = 96,485 C/(mol e⁻)

Faraday’s constant

N_A

N_A = 6.022 × 10²³ mol⁻¹

Avogadro’s number

h = 6.626 × 10⁻³⁴ J·s

Planck’s constant

c = 2.998 × 10⁸ m/s

Speed of light

k_B

k_B = 1.38 × 10⁻²³ J/K

Boltzmann’s constant

K_w

K_w = 1.0 × 10⁻¹⁴

Ion-product constant for water at 25 °C

STP

273.15 K and 1 atm

Standard temperature and pressure

Pressure

1 atm = 760 mm Hg = 760 torr = 101.325 kPa

Pressure unit conversions

c_water

c = 4.18 J/(g·°C)

Specific heat capacity of liquid water

Practise with the sheet open Exam-skill drills Cheatsheet