What is meant by the relative atomic mass of an element?

It is the average mass of its atoms compared with one twelfth of the mass of a carbon-12 atom.

What is meant by the relative molecular mass of a substance?

It is the sum of the relative atomic masses of all the atoms in one molecule of that substance.

How does relative formula mass differ from relative molecular mass?

Relative formula mass is used for substances that are not made of molecules, such as ionic compounds, and is the sum of the relative atomic masses shown in the formula unit.

Calculate the relative formula mass of CO₂, given relative atomic masses of carbon 12 and oxygen 16.

It is 44, because 12 + (2 × 16) = 44.

Calculate the relative formula mass of Ca(OH)₂, given relative atomic masses of calcium 40, oxygen 16 and hydrogen 1.

It is 74, because 40 + 2 × (16 + 1) = 74.

A compound has the formula XO₂ and a relative formula mass of 60. Oxygen has a relative atomic mass of 16. What is the relative atomic mass of X?

It is 28, because 60 − (2 × 16) = 28.

OCR GCSE Combined Science Chemistry J250

Chemical Reactions

Topic overview Writing formulae of elements and compounds Balanced equations, and deducing formulae from ions State symbols in equations Conservation of mass, and changes of mass in open systems Exothermic and endothermic reactions, and reaction profiles Activation energy and why reactions need it Oxidation and reduction in terms of oxygen Acids, alkalis and neutralisation The ionic equation for neutralisation, and reactions of acids The pH scale, acidity and alkalinity Measuring pH with indicators and pH meters Products at the electrodes, and molten ionic compounds Electrolysis of aqueous solutions Inert and non-inert electrodes

Elements, Compounds and Mixtures

Topic overview Purity, and melting point data Relative formula mass and empirical formulae Formulations, and separating mixtures Paper and thin layer chromatography Interpreting chromatograms and Rf values Choosing a purification method Metals and non-metals in the Periodic Table Electron arrangement and position in the Periodic Table Ionic, covalent and metallic bonding compared Dot and cross diagrams, and the limits of models Reactions, electron arrangement and Mendeleev's table Carbon's four bonds, and the variety of organic compounds Diamond, graphite, fullerenes and graphene Bond strength, intermolecular forces and predicting state Bulk properties of ionic, molecular and giant structures

Global Challenges

Topic overview Extracting metals by reduction and by electrolysis Life cycle assessment Recycling, and the decisions behind it Fractional distillation of crude oil The fractions, and crude oil as a feedstock Dependence on hydrocarbons, and cracking The Earth's early atmosphere, and how oxygen developed The greenhouse effect, and evidence for climate change Effects of carbon dioxide and methane, and air pollutants Producing potable water

Monitoring and Controlling Chemical Reactions

Topic overview Measuring the rate of a reaction, and rate graphs Factors affecting rate, and collision theory Surface area and rate Catalysts and how they work Reversible reactions and dynamic equilibrium Activation energy, and enzymes as catalysts

Particles

Topic overview The particle model and changes of state How the atomic model changed, and the structure of the atom The size of atoms, and the subatomic particles Calculating protons, neutrons and electrons

Predicting and Identifying Reactions

Topic overview Groups 1, 7 and 0, and why they behave as they do Predicting reactivity from the Periodic Table Reactivity of metals with water and acids Tests for oxygen, hydrogen, carbon dioxide and chlorine

Elements, Compounds and Mixtures

Relative formula mass and empirical formulae

3 things to know
Paper 3 (Chemistry)

Overview

What relative atomic, molecular and formula mass mean, calculating relative formula masses such as CO₂ and Ca(OH)₂, why the totals balance on both sides of an equation, what an empirical formula is, and working out empirical and molecular formulae from molecules, models, diagrams and relative formula mass.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Why must the total relative formula mass of the reactants equal that of the products in a balanced equation?

Key term

Each molecule of a compound contains 6 carbon atoms and 12 hydrogen atoms. What is its empirical formula?

Key term

A molecule contains 2 nitrogen atoms and 4 oxygen atoms. What is its empirical formula?

Questions from this subtopic

Each one with the model answer underneath.

1
What is meant by the relative atomic mass of an element?
Model answer
It is the average mass of its atoms compared with one twelfth of the mass of a carbon-12 atom.
2
What is meant by the relative molecular mass of a substance?
Model answer
It is the sum of the relative atomic masses of all the atoms in one molecule of that substance.
3
How does relative formula mass differ from relative molecular mass?
Model answer
Relative formula mass is used for substances that are not made of molecules, such as ionic compounds, and is the sum of the relative atomic masses shown in the formula unit.
4
Calculate the relative formula mass of CO₂, given relative atomic masses of carbon 12 and oxygen 16.
Model answer
It is 44, because 12 + (2 × 16) = 44.
5
Calculate the relative formula mass of Ca(OH)₂, given relative atomic masses of calcium 40, oxygen 16 and hydrogen 1.
Model answer
It is 74, because 40 + 2 × (16 + 1) = 74.
6
A compound has the formula XO₂ and a relative formula mass of 60. Oxygen has a relative atomic mass of 16. What is the relative atomic mass of X?
Model answer
It is 28, because 60 − (2 × 16) = 28.

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Chemical Reactions

Elements, Compounds and Mixtures

Global Challenges

Monitoring and Controlling Chemical Reactions

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AMetals are on the left and in the centre, and non-metals are on the right-hand side.

BKI, because potassium loses one electron and iodine gains one.

CCaCl₂, because a calcium atom loses two electrons and each chlorine atom gains one.

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