Pearson Edexcel International GCSE in Chemistry · 4CH1 · Free · no account needed
Chemistry equations and formulas
Every calculation formula on the Pearson Edexcel International GCSE Chemistry (4CH1) course — 17 of them, each with what its symbols stand for and the units the examiner expects, in specification order and linked to the lesson that teaches it.
These are the relationships behind the calculations the specification examines — moles, concentration, gas volumes, yields, energetics — on one sheet. There is no formula sheet in the Chemistry exam: every one of these must be recalled, which is why the list is here in one place. A formula you can write is not yet one you can use: each lesson below puts it to work in a worked example, then in five marked questions. Learn the list here; then open the lesson, prove it, teach it back to an AI pupil and defend it. Completely free.
Topic 1 · Principles of chemistry
| Equation | Where | Units | Lesson |
|---|---|---|---|
| Rf = distance moved by the spot ÷ distance moved by the solvent | Both measured from the pencil baseline; the spot distance is to the CENTRE of the spot. | None — it is a ratio of two lengths. Between 0 and 1, and never above 1. | Pure Substances, Mixtures and How to Separate Them |
| Ar = Σ(isotope mass × % abundance) ÷ 100 | Convert a ratio to percentages first. For an unknown abundance x: Ar × 100 = m₁x + m₂(100 − x) | None — Ar is a ratio. | Inside the Atom |
| solubility = (mass of solute ÷ mass of water) × 100 | mass of solute and mass of water both in grams, at a stated temperature | g per 100 g of water. Rearranged: mass of solute = (solubility × mass of water) ÷ 100. | Solubility and Solubility Curves |
| amount (mol) = mass (g) ÷ Mr | Mr is the relative formula mass, or Ar for an element existing as atoms | mol, g, and Mr has no units. Rearranged: mass = amount × Mr, and Mr = mass ÷ amount. | The Mole |
| number of particles = amount (mol) × 6.02 × 10²³ | 6.02 × 10²³ per mole is the Avogadro constant; the particles may be atoms, molecules or ions depending on the substance | mol, and the answer is a plain count with no unit. | The Mole |
| moles = mass ÷ relative formula mass | mass in grams (g), relative formula mass has no units | mol, g. Rearranged: mass = moles × relative formula mass. This converts between what a balance measures and what an equation counts. | Reacting Masses and Percentage Yield |
| percentage yield = (actual yield ÷ theoretical yield) × 100 | actual yield is the mass obtained in the experiment; theoretical yield is the mass calculated from the balanced equation | Both masses in the same unit, which cancels; the answer is a percentage. A value above 100% means the product was not dry. | Reacting Masses and Percentage Yield |
| moles = concentration × volume | concentration in moles per cubic decimetre (mol/dm³), volume in cubic decimetres (dm³) | mol, mol/dm³, dm³. Rearranged: concentration = moles ÷ volume. Convert cm³ to dm³ by DIVIDING BY 1000. | Concentration and Gas Volumes |
| volume of gas = moles × 24 | volume in cubic decimetres (dm³), 24 dm³ is the molar volume at room temperature and pressure | dm³, mol. Rearranged: moles = volume ÷ 24. Applies at rtp only, and is the same for every gas. | Concentration and Gas Volumes |
Topic 2 · Inorganic chemistry
| Equation | Where | Units | Lesson |
|---|---|---|---|
| percentage of oxygen = (decrease in volume ÷ original volume) × 100 | decrease in volume is the original volume minus the final volume; both volumes in the same units | The units cancel, so cm³ throughout is fine. The decrease IS the oxygen, because only oxygen has been removed. | The Gases in the Air |
Topic 3 · Physical chemistry
| Equation | Where | Units | Lesson |
|---|---|---|---|
| Q = mcΔT | Q is the heat energy in joules (J), m is the mass of the SOLUTION in grams (g), c is the specific heat capacity in J/g/°C, ΔT is the temperature change in °C | J, g, J/g/°C, °C. Take c as 4.18 J/g/°C for dilute aqueous solutions, and the density as 1 g/cm³ so that volume in cm³ gives mass in g. | Calorimetry Calculations |
| ΔH = Q ÷ moles | ΔH is the molar enthalpy change in kilojoules per mole (kJ/mol), Q is the heat energy in kilojoules (kJ), moles is the amount of the specified reactant | kJ/mol, kJ, mol. Convert Q from J to kJ by dividing by 1000. EXOTHERMIC is NEGATIVE; ENDOTHERMIC is POSITIVE. | Calorimetry Calculations |
| ΔH = bonds broken − bonds made | bonds broken is the total bond energy of all bonds in the reactants; bonds made is the total for all bonds in the products | kJ/mol throughout. A NEGATIVE answer is exothermic; a POSITIVE answer is endothermic. Never reverse the subtraction. | Energy Diagrams and Bond Energies |
| mean rate of reaction = quantity of product formed ÷ time taken | quantity as a volume of gas, a mass, or an amount in moles; the rate at one instant is the gradient of a tangent to the curve; for a timed end point, rate = 1 ÷ time | cm³/s, g/s or mol/s; 1/s (s⁻¹) for 1 ÷ time | Rates of Reaction |
Topic 4 · Organic chemistry
| Equation | Where | Units | Lesson |
|---|---|---|---|
| CₙH₂ₙ₊₂ | n is the number of carbon atoms in the molecule | No units — a formula relating atom counts, not a measured quantity | Alkanes |
| CₙH₂ₙ | n is the number of carbon atoms, and n is at least 2 because a C=C bond needs two carbons | No units — a formula relating atom counts, not a measured quantity | Alkenes |
| CₙH₂ₙ₊₁OH | n is the number of carbon atoms in the chain carrying the –OH group | No units — a general formula relating atom counts, not a measured quantity | Alcohols |
From the equation to the mark
Each lesson linked above explains where the equation comes from, shows it worked, and then asks you to prove it, teach it and defend it — against Lavoisier, Dalton or Avogadro, inside Incandio.
- Every Chemistry lesson — Chemistry on Incandio, topic by topic.
- Chemistry key terms — every definition to learn word for word.
- Chemistry required practicals — method, variables and errors for every one.
- The 4CH1 specification · Incandio Science