Chemistry

Reactions, solutions, gases and thermodynamics

Chemistry formulas help you connect what happens in a beaker, flask, or reaction vessel to the numbers behind it, from gases and solutions to acids, rates, and energy changes. This category is useful for students, exam prep, and anyone doing lab or industry work, because these formulas make it easier to calculate concentrations, predict reactions, and understand how and why chemical systems behave the way they do.

Ideal Gas Law

PV = nRT

Relates pressure, volume, moles and temperature of a gas.

Molarity

M = \frac{n}{V}

Concentration in moles of solute per litre of solution.

Moles from Mass

n = \frac{m}{M}

Number of moles from mass and molar mass.

pH

pH = -\log_{10}[H^+]

Acidity from hydrogen-ion concentration.

pOH

pOH = -\log_{10}[OH^-]

Basicity from hydroxide-ion concentration.

Henderson–Hasselbalch

pH = pK_a + \log\frac{[A^-]}{[HA]}

pH of a buffer solution.

Dilution Equation

C_1 V_1 = C_2 V_2

Relates concentration and volume before and after dilution.

Rate Law

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

Reaction rate as a function of reactant concentrations.

Arrhenius Equation

k = A e^{-E_a/RT}

Temperature dependence of the reaction rate constant.

Gibbs Free Energy

\Delta G = \Delta H - T\Delta S

Predicts spontaneity of a reaction.

Nernst Equation

E = E^\circ - \frac{RT}{nF}\ln Q

Cell potential under non-standard conditions.

Beer–Lambert Law

A = \varepsilon l c

Absorbance of light by a solution.

Equilibrium Constant

K = \frac{[C]^c[D]^d}{[A]^a[B]^b}

Ratio of product to reactant concentrations at equilibrium.

Combined Gas Law

\frac{P_1 V_1}{T_1} = \frac{P_2 V_2}{T_2}

Relates two states of a fixed amount of gas.

Percent Yield

\% = \frac{\text{actual}}{\text{theoretical}} \times 100

Efficiency of a chemical reaction.

Avogadro's Number

N = n \times N_A

Number of particles from moles.

Density

\rho = \frac{m}{V}

Mass per unit volume of a substance.

Percent Composition

\% = \frac{\text{mass of element}}{\text{molar mass}} \times 100

Mass percentage of an element in a compound.

Boyle's Law

P_1 V_1 = P_2 V_2

Pressure and volume are inversely related at constant temperature.

Charles's Law

\frac{V_1}{T_1} = \frac{V_2}{T_2}

Volume is proportional to temperature at constant pressure.

Ideal Gas Law

PV = nRT

Relates pressure, volume, moles, and temperature of a gas.

Boyle Law

P_1 V_1 = P_2 V_2

At constant temperature, pressure and volume are inversely related.

Charles Law

\frac{V_1}{T_1} = \frac{V_2}{T_2}

At constant pressure, volume is proportional to temperature.

Gay-Lussac Law

\frac{P_1}{T_1} = \frac{P_2}{T_2}

At constant volume, pressure is proportional to temperature.

Avogadro Number

N_A = 6.022\times 10^{23}

Number of particles in one mole.

Number of Particles

N = n\times N_A

Particles from moles and Avogadro number.

Gas Density

\rho = \frac{PM}{RT}

Density of an ideal gas.

Rate Law

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

Reaction rate as a function of reactant concentrations.

First-Order Half-Life

t_{1/2} = \frac{0.693}{k}

Half-life of a first-order reaction.

Mass Percent

\% = \frac{m_{solute}}{m_{solution}}\times 100

Concentration as a percentage by mass.

Molality

b = \frac{n_{solute}}{m_{solvent}}

Moles of solute per kilogram of solvent.

Combined Gas Law

\frac{P_1 V_1}{T_1} = \frac{P_2 V_2}{T_2}

Combines Boyle, Charles, and Gay-Lussac laws.

Enthalpy Change

\Delta H = H_{products} - H_{reactants}

Heat absorbed or released at constant pressure.

Hess Law

\Delta H = \sum \Delta H_{steps}

Total enthalpy change is the sum of step changes.

pH and pOH

pH + pOH = 14

Relationship between acidity and basicity at 25°C.

Water Ion Product

K_w = [H^+][OH^-] = 10^{-14}

Self-ionization constant of water at 25°C.

Photosynthesis

6\,CO_2 + 6\,H_2O \;\xrightarrow{\text{light}}\; C_6H_{12}O_6 + 6\,O_2

The overall reaction by which plants use light energy to convert carbon dioxide and water into glucose and oxygen.

Combustion of a Hydrocarbon

C_xH_y + \left(x + \tfrac{y}{4}\right)O_2 \rightarrow x\,CO_2 + \tfrac{y}{2}\,H_2O

Complete combustion of a hydrocarbon in oxygen, releasing energy and producing carbon dioxide and water.