3.1

Formulae

Cambridge IGCSE Chemistry (0620)  · Unit 3: Stoichiometry  · 12 flashcards

Formulae is topic 3.1 in the Cambridge IGCSE Chemistry (0620) syllabus , positioned in Unit 3 — Stoichiometry , alongside Equations and Calculations.  In one line: The molecular formula shows the number and type of different atoms in one molecule of a compound.

This topic is examined in Paper 1 (multiple-choice) and Papers 3/4 (theory), plus Paper 5 or Paper 6 (practical / alternative to practical). Past papers from 2022 to 2025 show this topic across undefined questions worth 381 marks (around 6.0% of all Chemistry marks in those years).

The deck below contains 12 flashcards — 6 definitions, 1 key concept and 4 identification cards — covering the precise wording mark schemes reward.  Use the 6 definition cards to lock down command-word answers (define, state), then move on to the concept and application cards to handle explain, describe and compare questions.

Key definition

Molecular formula

The molecular formula shows the number and type of different atoms in one molecule of a compound.

Example: The molecular formula for glucose is C₆H₁₂O₆, indicating 6 carbon, 12 hydrogen, and 6 oxygen atoms.

What the Cambridge 0620 syllabus says

2026-2028 syllabus, in our words

What the syllabus expects you to be able to do in this topic, in plain English. Match the command word (Describe, Explain, State, etc.) in your answer to score full marks. For the exact wording, see the official syllabus (PDF).

  1. State State the formulae of all elements and compounds named anywhere in the subject content.
  2. Define Define a compound's molecular formula: the number and kind of each different atom in one molecule.
  3. Deduce Deduce a simple compound's formula from a model or diagram by using the relative numbers of atoms shown.
  4. Construct Construct both word and symbol equations, including state symbols, to show reactants turning into products.
  5. Define Define a compound's empirical formula as the ratio of its different atoms or ions, in the simplest whole numbers. Supplement
  6. Deduce Deduce an ionic compound's formula from the charges on its ions, or from the relative numbers of ions shown in a model or diagram. Supplement
  7. Construct Construct ionic equations and other symbol equations, all with state symbols. Supplement
  8. Deduce Deduce a reaction's symbol equation, including state symbols, when given the relevant information. Supplement
Definition Flip

Define molecular formula.

Answer Flip

The molecular formula shows the number and type of different atoms in one molecule of a compound.

Example: The molecular formula for glucose is C₆H₁₂O₆, indicating 6 carbon, 12 hydrogen, and 6 oxygen atoms.
Definition Flip

Define empirical formula.

Answer Flip

The empirical formula shows the simplest whole number ratio of the different atoms or ions in a compound.

Example: The empirical formula for glucose (C₆H₁₂O₆) is CH₂O.
Definition Flip

What does a symbol equation show?

Answer Flip

A symbol equation shows how reactants form products using chemical symbols and formulae, including state symbols (s, l, g, aq).

Example: 2H₂(g) + O₂(g) → 2H₂O(l).
Definition Flip

What are the standard state symbols and what do they represent?

Answer Flip

The state symbols are (s) for solid, (l) for liquid, (g) for gas, and (aq) for aqueous solution (dissolved in water).

Example: NaCl(s) means solid sodium chloride.
Key Concept Flip

How do you deduce the formula of an ionic compound from charges on the ions?

Answer Flip

The overall charge of an ionic compound must be neutral. Use the charges to balance the ions.

Example: Aluminum (Al³⁺) and Oxygen (O²⁻) combine to form Al₂O₃ (2 x +3 = +6, 3 x -2 = -6).
Key Concept Flip

What is the formula for carbon dioxide?

Answer Flip

The formula for carbon dioxide is CO₂. One carbon atom covalently bonded to two oxygen atoms. It is a greenhouse gas.

Key Concept Flip

What is the formula for methane?

Answer Flip

The formula for methane is CH₄. One carbon atom covalently bonded to four hydrogen atoms. It is the main component of natural gas.

Key Concept Flip

What is the formula for water?

Answer Flip

The formula for water is H₂O. Two hydrogen atoms covalently bonded to one oxygen atom. It is essential for life.

Key Concept Flip

Give three examples of a Calcium compound.

Answer Flip

Three examples of calcium compounds are calcium hydroxide (Ca(OH)₂), lime (CaO) and limestone (CaCO₃). Calcium is a group 2 metal.

Definition Flip

What are the conditions at equilibrium?

Answer Flip

At equilibrium, the rate of forward reaction = rate of reverse reaction, and the concentrations of reactants and products are constant/unchanging. The reaction is dynamic.

Key Concept Flip

How do you calculate the number of moles, given volume and concentration?

Answer Flip

Moles = (Volume in cm³/1000) * Concentration in mol/dm³.

Example: 50.0 cm³ of 0.200 mol/dm³ HCl gives moles of HCl = (50.0/1000) * 0.200 = 0.01 mol.
Definition Flip

State the effect of carbon monoxide on the body.

Answer Flip

Carbon monoxide reduces the oxygen-carrying capacity of the blood. This causes breathing problems, drowsiness, headaches, sickness, or even death.

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2.4 Structure and properties 3.2 Equations

Key Questions: Formulae

Define molecular formula.

The molecular formula shows the number and type of different atoms in one molecule of a compound.

Example: The molecular formula for glucose is C₆H₁₂O₆, indicating 6 carbon, 12 hydrogen, and 6 oxygen atoms.
Define empirical formula.

The empirical formula shows the simplest whole number ratio of the different atoms or ions in a compound.

Example: The empirical formula for glucose (C₆H₁₂O₆) is CH₂O.
What does a symbol equation show?

A symbol equation shows how reactants form products using chemical symbols and formulae, including state symbols (s, l, g, aq).

Example: 2H₂(g) + O₂(g) → 2H₂O(l).
What are the standard state symbols and what do they represent?

The state symbols are (s) for solid, (l) for liquid, (g) for gas, and (aq) for aqueous solution (dissolved in water).

Example: NaCl(s) means solid sodium chloride.
What are the conditions at equilibrium?

At equilibrium, the rate of forward reaction = rate of reverse reaction, and the concentrations of reactants and products are constant/unchanging. The reaction is dynamic.

Tips to avoid common mistakes in Formulae

More topics in Unit 3 — Stoichiometry

Formulae sits alongside these Chemistry decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.

3.2 Equations

10 flashcards
3.3 Calculations

16 flashcards

Cambridge syllabus keywords to use in your answers

These are the official Cambridge 0620 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.

formula chemical formula molecular formula empirical formula displayed formula structural formula valency ion charge polyatomic ion sulfate nitrate carbonate hydroxide ammonium

Key terms covered in this Formulae deck

Every term below is defined in the flashcards above. Use the list as a quick recall test before your exam — if you can't define one of these in your own words, flip back to that card.

Molecular formula
Empirical formula
What does a symbol equation show
The standard state symbols and what do they represent
The conditions at equilibrium
Effect of carbon monoxide on the body

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How to study this Formulae deck

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