IGCSE Chemistry 0620 — Topic 4
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Chemical Formulae & Equations

Moles, Relative Mass & Stoichiometry

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Learning Objectives

Write and balance chemical equations

Calculate relative formula mass (Mr)

Understand the mole concept

Use moles in calculations

Calculate concentration from moles

Solve stoichiometry problems

Relative Formula Mass (Mr)

Definition: Sum of relative atomic masses of all atoms in a formula.

H₂O: (2 × 1) + 16 = 18
CaCO₃: 40 + 12 + (3 × 16) = 100
NaOH: 23 + 16 + 1 = 40

The Mole Concept

Mole (mol): Unit of amount of substance; defined by Avogadro's number.

1 mole = 6.02 × 10²³ particles
Molar mass (g/mol) numerically equals Mr
Example: 1 mole of H₂O = 18 g

Converting Between Mass and Moles

Number of moles = mass (g) ÷ molar mass (g/mol)
moles = m/M
m = n × M
Example: How many moles in 36 g of H₂O?
n = 36 ÷ 18 = 2 moles

Chemical Equations

Chemical equation: Shows what happens in a chemical reaction using symbols.

Reactants → Products
2H₂ + O₂ → 2H₂O
Numbers (coefficients) must be whole numbers

Balancing Chemical Equations

Rules:

Count atoms of each element on both sides

Use coefficients (whole numbers in front) to balance

Never change subscripts (that changes the compound)

Check each element is balanced

Moles and Reactions

Use balanced equations to find mole ratios:

From: 2H₂ + O₂ → 2H₂O
Mole ratio: H₂ : O₂ : H₂O = 2 : 1 : 2
If you have 4 moles H₂, you need 2 moles O₂

Stoichiometry Problems

Use balanced equation to find mole ratios

Convert mass → moles using molar mass

Use mole ratios to find moles of product

Convert moles → mass of product

Ionic Equations

Ionic equation: Shows only the particles that actually react (spectator ions removed).

Full: 2HCl + Ca(OH)₂ → CaCl₂ + 2H₂O
Ionic: H⁺ + OH⁻ → H₂O (universal for all acids + bases)

Concentration (Molarity)

Molarity = moles of solute ÷ volume of solution (dm³)

c = n/V
Example: 2 moles in 500 cm³ (0.5 dm³) = 4 mol/dm³
Also: c = (mass in g / Mr) / volume (dm³)

Concentration Calculations

If you know: mass of solute, Mr, and volume
1. Calculate moles: n = m/Mr
2. Calculate concentration: c = n/V

Lesson Summary

Mr = sum of all atomic masses in formula

1 mole = 6.02 × 10²³ particles

Molar mass (g/mol) numerically equals Mr

Balance equations using coefficients

Use mole ratios from equations for stoichiometry

Convert between mass, moles, and volume

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