3.2

Equations

Cambridge IGCSE Chemistry (0620)  · Unit 3: Stoichiometry  · 10 flashcards

Equations is topic 3.2 in the Cambridge IGCSE Chemistry (0620) syllabus , positioned in Unit 3 — Stoichiometry , alongside Formulae and Calculations.  In one line: Ar is the average mass of the isotopes of an element compared to 1/12th of the mass of an atom of carbon-12.

This topic is examined in Paper 1 (multiple-choice) and Papers 3/4 (theory), plus Paper 5 or Paper 6 (practical / alternative to practical).

The deck below contains 10 flashcards — 4 definitions, 2 key concepts and 2 identification cards — covering the precise wording mark schemes reward.  Use the 4 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

Relative atomic mass (Ar)

Ar is the average mass of the isotopes of an element compared to 1/12th of the mass of an atom of carbon-12.

Example: the Ar of chlorine is approximately 35.5, considering the abundance of chlorine-35 and chlorine-37.

What the Cambridge 0620 syllabus says

Official 2026-2028 spec

These are the exact learning objectives Cambridge sets for this topic. Match the command word (Describe, Explain, State, etc.) in your answer to score full marks.

  1. Describe Describe relative atomic mass, A,, as the average mass of the isotopes of an element compared to 1/12th of the mass of an atom of 12C
  2. Define Define relative molecular mass, M., as the sum of the relative atomic masses. Relative formula mass, M., will be used for ionic compounds
  3. Calculate Calculate reacting masses in simple proportions. Calculations will not involve the mole concept
Definition Flip

Define relative atomic mass (Ar).

Answer Flip

Ar is the average mass of the isotopes of an element compared to 1/12th of the mass of an atom of carbon-12.

Example: the Ar of chlorine is approximately 35.5, considering the abundance of chlorine-35 and chlorine-37.
Definition Flip

Define relative molecular mass (Mr).

Answer Flip

Mr is the sum of the relative atomic masses of the atoms in a molecule.

Example: the Mr of water (H₂O) is (2 x Ar of H) + Ar of O = (2 x 1) + 16 = 18.
Definition Flip

Define relative formula mass.

Answer Flip

The relative formula mass is used for ionic compounds. It is calculated by summing the relative atomic masses of each element in the formula.

Example: the relative formula mass of NaCl is Ar(Na) + Ar(Cl) = 23 + 35.5 = 58.5.
Key Concept Flip

What state symbols are commonly used in chemical equations, and what do they represent?

Answer Flip

(s) - solid, (aq) - aqueous (dissolved in water), (l) - liquid, (g) - gas. All state symbols must be correct in chemical equations. For

Example: NaCl(s) + H₂O(l) → Na⁺(aq) + Cl⁻(aq).
Key Concept Flip

In the reaction 2HCl + CaCO₃ → CaCl₂ + H₂O + CO₂, if 0.0100 moles of HCl react, how many moles of CO₂ are produced?

Answer Flip

According to the balanced equation, 2 moles of HCl produce 1 mole of CO₂. Therefore, 0.0100 moles of HCl produce 0.0100 / 2 = 0.00500 moles of CO₂.

Key Concept Flip

In the reaction 2HCl + CaCO₃ → CaCl₂ + H₂O + CO₂, if 0.0100 moles of HCl react, calculate the volume of CO₂ produced at room temperature and pressure where 1 mole of gas occupies 24000 cm³.

Answer Flip

First find moles of CO₂ produced, in this case 0.00500. Volume of CO₂ = 0.00500 x 24000 = 120 cm³.

Key Concept Flip

Give an example of a reaction that causes nitrogen oxides to form.

Answer Flip

Nitrogen and oxygen from the air react at high temperatures, such as in a car engine, to form nitrogen oxides.

Key Concept Flip

What are the negative effects of producing nitrogen oxides?

Answer Flip

Nitrogen oxides can cause acid rain and respiratory problems.

Definition Flip

What type of reaction is the breakdown of a compound by heating?

Answer Flip

This is called thermal decomposition. An example is the thermal decomposition of calcium carbonate into calcium oxide and carbon dioxide.

Key Concept Flip

Describe a test for ammonia gas.

Answer Flip

Add aqueous sodium hydroxide/ add damp red litmus paper. If ammonia is present, the litmus paper will turn blue.

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3.1 Formulae 3.3 Calculations

Key Questions: Equations

Define relative atomic mass (Ar).

Ar is the average mass of the isotopes of an element compared to 1/12th of the mass of an atom of carbon-12.

Example: the Ar of chlorine is approximately 35.5, considering the abundance of chlorine-35 and chlorine-37.
Define relative molecular mass (Mr).

Mr is the sum of the relative atomic masses of the atoms in a molecule.

Example: the Mr of water (H₂O) is (2 x Ar of H) + Ar of O = (2 x 1) + 16 = 18.
Define relative formula mass.

The relative formula mass is used for ionic compounds. It is calculated by summing the relative atomic masses of each element in the formula.

Example: the relative formula mass of NaCl is Ar(Na) + Ar(Cl) = 23 + 35.5 = 58.5.
What type of reaction is the breakdown of a compound by heating?

This is called thermal decomposition. An example is the thermal decomposition of calcium carbonate into calcium oxide and carbon dioxide.

More topics in Unit 3 — Stoichiometry

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

3.1 Formulae

12 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.

word equation symbol equation balanced equation state symbol reactant product coefficient conservation of mass

Key terms covered in this Equations 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.

Relative atomic mass (Ar)
Relative molecular mass (Mr)
Relative formula mass
What type of reaction is the breakdown of a compound by heating

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How to study this Equations deck

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