Who this is for: Cambridge IGCSE Chemistry (0620) students who want relative atomic mass (Ar) and relative molecular mass (Mr) — reading the periodic table, calculating Mr and linking mass to moles — to become reliable marks instead of arithmetic errors.
What query it owns: how to understand and revise relative masses of atoms and molecules in Cambridge IGCSE Chemistry.
Why this is safe: this page owns the relative masses revision-guide angle, while Tutopiya’s Relative Masses of Atoms and Molecules subtopic page owns the learning resource and the free Relative Masses quiz owns the practice.

Relative atomic mass and relative molecular mass are the foundation of every stoichiometry calculation in Cambridge IGCSE Chemistry (0620). Examiners expect you to read Ar from the periodic table, calculate Mr from formulae, and use these values in mole and reacting-mass questions. This guide covers the syllabus definitions, the calculation shortcuts that save time, and the question types that appear every year.

Key takeaways

• Relative atomic mass (Ar) compares an atom’s mass to ¹⁄₁₂ of a carbon-12 atom — read it from the periodic table.
• Relative molecular mass (Mr) is the sum of Ar values for all atoms in a formula.
• Relative formula mass is the same idea for ionic compounds — e.g. Mr of NaCl = 23 + 35.5 = 58.5.
• Use brackets correctly when counting atoms — Ca(OH)₂ has Mr = 40 + 2×(16+1) = 74.
• Mr links directly to moles: mass (g) = Mr × amount of substance (mol).

What are relative masses in Cambridge IGCSE Chemistry?

Relative atomic mass (Ar) is the average mass of an atom of an element compared to ¹⁄₁₂ of the mass of a carbon-12 atom. Relative molecular mass (Mr) is the sum of the relative atomic masses of all atoms shown in a molecular or ionic formula. These are dimensionless numbers used to convert between mass in grams and amount of substance in moles.

You can read the full explanation, periodic table use and worked examples on Tutopiya’s Relative Masses of Atoms and Molecules subtopic page before you attempt questions.

The core ideas you must master

Idea	What it means	How the exam uses it
Ar from periodic table	Mass number (weighted average) for each element	”State the Ar of chlorine.”
Mr calculation	Add Ar × atom count for each element	”Calculate the Mr of sulfuric acid.”
Brackets in Mr	Multiply subscript outside bracket by inside	”Calculate Mr of aluminium sulfate.”
Reacting masses	Use ratio of Mr values from equation	”Calculate the mass of product formed.”
Percentage composition	(Ar × n / Mr) × 100	”Calculate the % of oxygen in CO₂.”

How to calculate Mr — step by step

1. Write the correct formula — wrong formula means wrong Mr.
2. List each element and how many atoms of each (watch brackets).
3. Look up Ar values from the periodic table (use values given if supplied).
4. Add Ar × count for every element.
5. State Mr with no units — it is a relative value.

Once you have worked through a few, test yourself with the free Relative Masses quiz — it tells you fast whether the method has actually stuck.

Ar vs Mr vs reacting mass: which calculation does the question want?

Situation	What to do	Typical signal words
Mr of a compound	Sum Ar values from formula	”Calculate the relative molecular mass of…”
Mass from moles	mass = Mr × n	”Calculate the mass of 0.5 mol of…”
Reacting mass	Use molar ratio from balanced equation	”What mass of magnesium oxide is formed from 12 g of magnesium?”
% by mass of an element	(element mass in formula / Mr) × 100	”Calculate the percentage of nitrogen in ammonia.”

Relative masses in past-paper wording: command words that matter

Command word / phrase	What the question wants	Typical relative mass stem
Calculate	Show numerical working	”Calculate the Mr of calcium carbonate.”
State	Give Ar or Mr if known	”State the Ar of sodium.”
Determine	Work out from data	”Determine the Mr of the hydrocarbon.”
Deduce	Infer from reacting masses	”Deduce the formula of the oxide.”
Show that	Prove a given mass value	”Show that 49 g of sulfuric acid contains 1 mol.”

Worked exam-style stems (how to answer the wording)

1. “Calculate the relative molecular mass of sulfuric acid, H₂SO₄.” (2×1) + 32 + (4×16) = 2 + 32 + 64 = 98. Mark-scheme reward: all atoms counted + correct sum.
2. “Calculate the percentage by mass of oxygen in carbon dioxide, CO₂.” O mass = 32; Mr = 44; % = (32/44) × 100 = 72.7%. Reward: correct Mr and percentage formula.
3. “12 g of magnesium reacts with oxygen to form magnesium oxide. Calculate the mass of magnesium oxide formed.” 2Mg + O₂ → 2MgO. Mg 24 g → MgO 40 g. 12 g Mg → (12/24) × 40 = 20 g MgO. Reward: molar ratio from equation applied.

When you can recognise the wording instantly, work the full set on the Stoichiometry topical past paper questions and the Relative Masses quiz to lock the method in.

How relative masses connect to the rest of Stoichiometry

Relative masses depend on correct Formulae and feed directly into The Mole and the Avogadro Constant via mass = Mr × n. Every titration and yield question uses Mr. When you are ready to mix topics, the Cambridge IGCSE Chemistry resource hub lets you move straight from a weak subtopic into the next.

Common mistakes students make

• Using atomic number instead of relative atomic mass from the periodic table.
• Forgetting to multiply by subscripts — especially inside brackets.
• Adding Mr values without a balanced equation in reacting-mass questions.
• Giving Mr with units (g) — Mr is dimensionless.
• Rounding Ar values too early and losing accuracy marks.

When you need more support

If Mr and reacting-mass questions keep tripping you up — especially multi-step calculations — work through the Stoichiometry topical past paper questions and the Relative Masses quiz to pinpoint the exact gap, then get focused help from a Cambridge IGCSE Chemistry tutor to fix it quickly.

Frequently asked questions

Are relative masses hard in Cambridge IGCSE Chemistry? No — the arithmetic is straightforward. Marks are lost when students miscount atoms in formulae or skip the molar ratio from the balanced equation.

What is the difference between Ar and Mr? Ar is for a single element; Mr is the total for all atoms in a compound’s formula.

Do I need to learn Ar values by heart? You will always have a periodic table in the exam — but know common values (H=1, C=12, O=16, Na=23, Cl=35.5) to save time.

How do I revise relative masses effectively? Practise Mr calculations daily, then link to mole questions via the Relative Masses quiz before tackling full stoichiometry stems.

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