The Mole Concept (College Board AP® Chemistry): Study Guide

The Mole Concept

• The mole (mol) is the SI unit for the amount of substance

• This unit is essential because atoms, ions, and molecules are incredibly small

  • This means that it is impossible to count them directly in a laboratory

• One mole (mol) of any substance contains a fixed number of particles

  • This value is known as Avogadro's number (N_A)

N_A = 6.022 x 10²³ mol^-1

• The relationship between the number of particles, the amount in moles (n), and Avogadro's number is shown by the equation:

number of constituent particles = N_A x n

• So, Avogadro's number (N_A) shows that one mole of any substance contains 6.022 x 10²³ particles

Specifying the type of particle

• It is important to specify the type of particle

• One mole of a substance can contain a different number of:

  • Atoms

  • Ions

  • Molecules / formula units

Iron, Fe

• A piece of iron is made of individual iron atoms

• So, one mole of iron contains:

6.022 x 10²³ x 1 mole = 6.022 x 10²³ Fe atoms

Water, H₂O

• Water is a covalent molecule

• One mole of water contains:

6.022 x 10²³ x 1 mole = 6.022 x 10²³ H₂O molecules

Counting atoms

• However, each H₂O molecule is made from:

  • 2 hydrogen atoms

  • 1 oxygen atom

• So, one mole of water contains:

• Hydrogen atoms

6.022 x 10²³ x 2 = 1.2044 x 10²⁴ H atoms

• Oxygen atoms

6.022 x 10²³ x 1 = 6.022 x 10²³ O atoms

• Total atoms

(1.2044 x 10²⁴) + (6.022 x 10²³) = 1.8066 x 10²⁴ total atoms

Aluminum sulfate, Al₂(SO₄)₃

• Aluminum sulfate is an ionic compound

• One mole of aluminum sulfate contains:

6.022 x 10²³ x 1 mole = 6.022 x 10²³ Al₂(SO₄)₃ formula units

Counting atoms

• However, each Al₂(SO₄)₃ is made from:

  • 2 aluminum atoms

  • 3 sulfur atoms

  • 12 oxygen atoms

• So, one mole of aluminum sulfate contains:

• Aluminum atoms

6.022 x 10²³ x 2 = 1.2044 x 10²⁴ Al atoms

• Sulfur atoms

6.022 x 10²³ x 3 = 1.8066 x 10²⁴ S atoms

• Oxygen atoms

6.022 x 10²³ x 12 = 7.2264 x 10²⁴ O atoms

• Total atoms

(1.2044 x 10²⁴ ) + (1.8066 x 10²⁴) + (7.2264 x 10²⁴) = 1.02374 x 10²⁵ total atoms

Counting ions

• Each Al₂(SO₄)₃ is made from:

  • 2 aluminum ions, Al³⁺

  • 3 sulfate ions, SO₄^2-

• So, one mole of aluminum sulfate contains:

• Aluminum ions

6.022 x 10²³ x 2 = 1.2044 x 10²⁴ Al³⁺ ions

• Sulfate ions

6.022 x 10²³ x 3 = 1.8066 x 10²⁴ SO₄^2- ions

• Total ions

(1.2044 x 10²⁴ ) + (1.8066 x 10²⁴) = 3.011 x 10²⁴ total ions