GCSE Chemistry Formula Sheet — Equations & Calculations (AQA)

Every formula and required equation for AQA GCSE Chemistry (8462) in one place, organised by topic so you can revise quickly and find what you need during last-minute review. This sheet is completely free — use it alongside our free GCSE Chemistry practice tests to check what you have learned.

Quantitative Chemistry

Formula / Equation	What it means / How it is used
Relative formula mass, M_r = sum of all the A_r values in the formula	Add up the relative atomic masses of every atom in the compound, e.g. M_r of CO₂ = 12 + (16 × 2) = 44.
Number of moles = mass (g) ÷ M_r	Converts a measured mass of a substance into the amount in moles.
Mass (g) = moles × M_r	Rearrangement of the moles equation, used to find the mass of a given number of moles.
Concentration (g/dm³) = mass (g) ÷ volume (dm³)	Finds how many grams of solute are dissolved in each dm³ of solution.
Concentration (mol/dm³) = moles ÷ volume (dm³)	Expresses concentration in moles per dm³; remember 1 dm³ = 1000 cm³.
Reacting masses (3 steps): 1) work out moles of the known substance, 2) use the ratio in the balanced equation, 3) convert moles back to mass	The standard method for calculating the mass of a product or reactant from a balanced equation.
Percentage yield = (actual mass ÷ theoretical mass) × 100	Compares the mass of product actually obtained with the maximum mass the calculation predicts.
Atom economy = (M_r of desired product ÷ sum of M_r of all products) × 100	Shows what percentage of the starting materials ends up as useful product; higher is more efficient.
Volume of gas (dm³) = moles × 24	At room temperature and pressure, one mole of any gas occupies 24 dm³.
Percentage of an element in a compound = (A_r × number of atoms ÷ M_r of compound) × 100	Finds the percentage by mass of one element in a compound, e.g. the % of nitrogen in a fertiliser.
Relative atomic mass = sum of (isotope abundance × mass number) ÷ 100	Calculates A_r from the percentages of an element’s isotopes.

Chemical Equations You Must Know

Formula / Equation	What it means / How it is used
Acid + metal → salt + hydrogen	Metals above hydrogen in the reactivity series fizz in acids and produce hydrogen gas (test: squeaky pop with a lit splint).
Acid + alkali → salt + water	A neutralisation reaction; the salt formed depends on which acid and alkali are used.
Acid + metal carbonate → salt + water + carbon dioxide	Carbonates fizz in acids as carbon dioxide is released (test: turns limewater milky).
Metal + oxygen → metal oxide	Metals tarnish or burn in air to form an oxide coating or powder.
Metal carbonate → metal oxide + carbon dioxide (on heating)	Thermal decomposition: heating a metal carbonate breaks it down into the oxide and carbon dioxide.
Hydrocarbon + oxygen → carbon dioxide + water	Complete combustion, which happens when a fuel burns in plenty of oxygen.
Hydrocarbon + limited oxygen → carbon monoxide and/or carbon + water	Incomplete combustion, which happens when oxygen is in short supply; carbon monoxide is toxic.
A more reactive metal displaces a less reactive metal from its compound	The basis of displacement reactions, used to place metals in order in the reactivity series.
Oxidation = gain of oxygen / loss of electrons; Reduction = loss of oxygen / gain of electrons (OIL RIG)	OIL RIG helps you remember: Oxidation Is Loss of electrons, Reduction Is Gain of electrons.

Energy Changes

Formula / Equation	What it means / How it is used
q = m c ΔT	The energy change equation used in calorimetry questions; for water, AQA uses energy change (J) = mass of water (g) × 4.2 × temperature change (°C).
Energy change = sum of bond energies broken − sum of bond energies formed	Uses bond energy data to calculate the overall energy change of a reaction; breaking bonds takes in energy, forming bonds releases it.
Exothermic: negative energy change, temperature of surroundings rises. Endothermic: positive energy change, temperature of surroundings falls	The sign of the energy change tells you the reaction type; exothermic reactions release energy, endothermic reactions take it in.

Rates of Reaction

Formula / Equation	What it means / How it is used
Mean rate of reaction = quantity of product formed ÷ time taken	The basic rate equation; the quantity can be mass, volume or moles.
Mean rate (gas) = volume of gas produced ÷ time taken	Used when a reaction gives off a gas collected in a gas syringe or over water.
Rate from a graph = gradient of the curve (draw a tangent and use its gradient)	The steeper the gradient, the faster the reaction; a tangent is needed when the curve is not a straight line.
Rate units: g/s, cm³/s or mol/s	The unit depends on how the quantity of product is measured — grams, cm³ or moles per second.

Electrolysis

Formula / Equation	What it means / How it is used
Cathode (negative electrode): positive metal or hydrogen ions gain electrons and are reduced, e.g. Cu²⁺ + 2e⁻ → Cu	Positive ions move to the cathode, pick up electrons and form the pure element.
Anode (positive electrode): negative ions lose electrons and are oxidised, e.g. 2Cl⁻ → Cl₂ + 2e⁻	Negative ions move to the anode, give up electrons and form the non-metal element.
Half equations show only the ions and electrons at one electrode	Balance the atoms first, then add electrons so the total charge is the same on both sides.
Electrolysis of aqueous solutions: hydrogen forms at the cathode if the metal is reactive; at the anode, a halogen forms if a halide is present, otherwise oxygen forms	Because water provides H⁺ and OH⁻ ions, the least reactive species is discharged at each electrode.

Acids & the pH Scale

Formula / Equation	What it means / How it is used
pH scale runs from 0 to 14: acids are below 7, neutral is 7, alkalis are above 7	pH measures how acidic or alkaline a solution is; universal indicator gives the approximate pH from its colour.
H⁺ + OH⁻ → H₂O	The ionic equation for every neutralisation between an acid and an alkali.
Indicator colours — litmus: red in acid, blue in alkali. Phenolphthalein: colourless in acid, pink in alkali. Methyl orange: red in acid, yellow in alkali	Indicators change colour at different pH values, so choose the right one for the titration or test you are doing.

Organic Chemistry Basics

Formula / Equation	What it means / How it is used
Alkanes general formula: C_nH_2n+2	Lets you work out the formula of any alkane from its number of carbon atoms.
Alkenes general formula: C_nH_2n	Alkenes have one double bond, so they contain two fewer hydrogen atoms than the matching alkane.
First four alkanes: methane CH₄, ethane C₂H₆, propane C₃H₈, butane C₄H₁₀	The first four members of the alkane homologous series, which you should be able to name and draw.
Functional groups: alcohols contain −OH; carboxylic acids contain −COOH	The functional group gives a homologous series its characteristic reactions, e.g. ethanol is C₂H₅OH.
Hydrocarbon + oxygen → carbon dioxide + water	Complete combustion of alkanes and other hydrocarbons when oxygen is plentiful.

Atomic Structure

Formula / Equation	What it means / How it is used
Mass number = number of protons + number of neutrons	The mass number is the top number in nuclear notation; subtract the atomic number from it to find the neutrons.
Number of electrons = number of protons (in a neutral atom)	Atoms have no overall charge, so the positive and negative charges must balance.
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons	Isotopes have identical chemical properties but different masses, which is why A_r is often not a whole number.