Formulae & Equations

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Edexcel IAL Chemistry — Unit 1

Formulae & Equations

Everything you need to go from "I've read it" to "I can actually do it" — moles, masses, equations, and formula calculations, explained properly.

💡 The Big Idea: Chemistry is really just careful counting — atoms can't be created, destroyed, or lost track of, so every formula and equation in this chapter is a tool for counting particles accurately, whether they're too small to see or too many to imagine.

Summary — What This Chapter Covers

Formulae & Mass: the difference between relative isotopic mass, relative atomic mass (A_r), and relative formula/molecular mass (M_r) — and how to calculate M_r from a formula.
Avogadro & the Mole: what a "mole" actually is, the Avogadro constant, and the core equation linking moles, mass, and molar mass.
Full & Ionic Equations: how to balance chemical equations, work out ionic compound formulae from charges, write ionic equations by cancelling spectator ions, and recognise reaction types (displacement, neutralisation, precipitation).
Calculating Formulae: finding empirical and molecular formulae from mass/percentage data, percentage composition by mass, water of crystallisation, and the ideal gas equation (PV = nRT).

Topic 1: Formulae & Mass

1Basic Terms You Must Know Cold

Before anything else clicks, you need rock-solid definitions of the basic vocabulary. These sound simple, but exam questions love testing the precise wording — "an atom that has become electrically charged" is NOT the same mark as "a charged particle" if the mark scheme wants the specific idea of gaining/losing electrons.

Atom

The smallest part of an element that can participate in a chemical reaction. Think of it as the smallest "chemically active" unit.

Element

A substance made of only one type of atom — it cannot be broken down into anything simpler by chemical means.

Ion

An atom (or group of atoms) that has become electrically charged by gaining or losing electrons.

Molecule

Two or more atoms chemically joined together (can be the same element, e.g. O₂, or different elements).

Compound

A substance made of two or more different elements chemically joined together.

Empirical formula

The smallest whole-number ratio of atoms of each element in a compound. Example: glucose's molecular formula is C₆H₁₂O₆, but its empirical formula is CH₂O.

Why "empirical" vs "molecular" matters Empirical formula is a ratio — it tells you the simplest proportion of atoms, but not necessarily how many atoms are actually in one molecule. Molecular formula is the real, actual count. Two very different compounds can share the same empirical formula (CH₂O is both formaldehyde AND part of the ratio for glucose), which is exactly why you sometimes need extra data (like relative molecular mass) to pin down the true molecular formula.

2Relative Isotopic Mass vs Relative Atomic Mass

This is one of the most commonly confused pairs in the whole chapter, so let's be really precise.

1/12th the mass of one atom of carbon-12

Relative Isotopic Mass

This is the mass of one specific isotope of an element, relative to 1/12 of a carbon-12 atom. For example, the accurate isotopic mass of nitrogen-14 is 14.00307401, which we round to 14.0 for A-level purposes.