Chemistry Formula Library

Formulas are useless without meaning. Each page explains what the equation says, defines every variable, states the assumptions and works an example.

flask Acids & Bases

Henderson–Hasselbalch Equation

pH = pKa + log([A-] / [HA])

Acid Dissociation Constant Ka

Ka = [H+] [A-] / [HA]

Ion Product of Water Kw

Kw = [H+] [OH-] = 1.0 x 10^-14 (at 25 C); pH + pOH = 14.00

pH Definition

pH = -log[H+]

pOH Definition

pOH = -log[OH-]

flask Redox & Electrochemistry

Standard Cell Potential

E(cell) = E(cathode) - E(anode), both as standard reduction potentials

Faraday's Law of Electrolysis

mass = (current x time x molar mass) / (n x Faraday constant); moles of electrons = I x t / F

Nernst Equation

E = E(standard) - (RT/nF) x ln Q ; at 25 C: E = E(standard) - (0.0592 V / n) x log Q

flask Chemical Equilibrium

Equilibrium Constant Kc

Kc = [C]^c [D]^d / [A]^a [B]^b

Solubility Product Constant Ksp

Ksp = [cation]^x [anion]^y

Reaction Quotient Qc

Qc = [C]^c [D]^d / [A]^a [B]^b (using current, non-equilibrium concentrations)

flask States of Matter & Gases

Boyle's Law

P1V1 = P2V2

Charles's Law

V1/T1 = V2/T2

Combined Gas Law

P1V1/T1 = P2V2/T2

Graham's Law of Effusion

rate1/rate2 = sqrt(M2/M1)

Ideal Gas Law

PV = nRT

Dalton's Law of Partial Pressures

P_total = P1 + P2 + ... ; Pi = Xi × P_total

flask General Chemistry

Molar Mass Formula

M = m/n

Mole–Particle Conversion (Avogadro's Number)

N = n × NA

Percent Composition of a Compound

% element = (n × atomic weight / molar mass) × 100%

flask Chemical Kinetics

Arrhenius Equation

k = Ae^(−Ea/RT) ; ln(k2/k1) = (Ea/R)(1/T1 − 1/T2)

First-Order Half-Life

t1/2 = ln2/k = 0.693/k

Integrated First-Order Rate Law

ln[A]t = −kt + ln[A]0

Rate Law

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

flask Math for Chemistry

Density Formula

density = mass / volume

Percent Yield

percent yield = (actual yield / theoretical yield) x 100%

Scientific Notation

a x 10^n, where 1 <= a < 10

flask Solutions & Solubility

Dilution Equation M1V1 = M2V2

M1 x V1 = M2 x V2

Freezing Point Depression

delta Tf = Kf x molality x i

Mass Percent Composition

mass percent = (mass of solute / mass of solution) x 100%

Molality Definition

molality = moles of solute / kg of solvent

Molarity Definition

Molarity = moles of solute / liters of solution

Mole Fraction

X(i) = moles of component i / total moles

flask Thermochemistry & Thermodynamics

Calorimetry: Heat Lost = Heat Gained

q_lost = -q_gained (heat lost by hot = heat gained by cold)

Standard Enthalpy of Reaction

ΔH°rxn = ΣnΔH°f(products) − ΣmΔH°f(reactants)

Entropy Change

ΔS = q_rev/T ; ΔS°rxn = ΣnS°(products) − ΣmS°(reactants)

Gibbs Free Energy

ΔG = ΔH − TΔS ; ΔG° = −RT ln K

Specific Heat Equation (q = mcΔT)

q = mcΔT

Hess's Law of Heat Summation

ΔH°rxn = Σ ΔH°(steps) — overall enthalpy = sum of step enthalpies